diff --git a/hott/init/tactic.hlean b/hott/init/tactic.hlean
index 42d8b8221..9ee30d92c 100644
--- a/hott/init/tactic.hlean
+++ b/hott/init/tactic.hlean
@@ -42,6 +42,7 @@ opaque definition id          : tactic := builtin
 opaque definition beta        : tactic := builtin
 opaque definition info        : tactic := builtin
 opaque definition whnf        : tactic := builtin
+opaque definition contradiction : tactic := builtin
 opaque definition rotate_left (k : num) := builtin
 opaque definition rotate_right (k : num) := builtin
 definition rotate (k : num) := rotate_left k
diff --git a/library/init/tactic.lean b/library/init/tactic.lean
index de1a049a0..685bfb58f 100644
--- a/library/init/tactic.lean
+++ b/library/init/tactic.lean
@@ -42,6 +42,7 @@ opaque definition id          : tactic := builtin
 opaque definition beta        : tactic := builtin
 opaque definition info        : tactic := builtin
 opaque definition whnf        : tactic := builtin
+opaque definition contradiction : tactic := builtin
 opaque definition rotate_left (k : num) := builtin
 opaque definition rotate_right (k : num) := builtin
 definition rotate (k : num) := rotate_left k
diff --git a/src/emacs/lean-syntax.el b/src/emacs/lean-syntax.el
index a6dc5178a..7a05882d7 100644
--- a/src/emacs/lean-syntax.el
+++ b/src/emacs/lean-syntax.el
@@ -135,7 +135,7 @@
                 "apply" "fapply" "rename" "intro" "intros" "all_goals" "fold"
                 "generalize" "generalizes" "clear" "clears" "revert" "reverts" "back" "beta" "done" "exact" "rexact"
                 "refine" "repeat" "whnf" "rotate" "rotate_left" "rotate_right" "inversion" "cases" "rewrite" "esimp"
-                "unfold" "change" "check_expr"))
+                "unfold" "change" "check_expr" "contradiction"))
            word-end)
       (1 'font-lock-constant-face))
      ;; Types
diff --git a/src/library/constants.cpp b/src/library/constants.cpp
index 0127b3769..4c738bff8 100644
--- a/src/library/constants.cpp
+++ b/src/library/constants.cpp
@@ -14,6 +14,8 @@ name const * g_bool_tt = nullptr;
 name const * g_char = nullptr;
 name const * g_char_mk = nullptr;
 name const * g_dite = nullptr;
+name const * g_empty = nullptr;
+name const * g_empty_rec = nullptr;
 name const * g_eq = nullptr;
 name const * g_eq_elim_inv_inv = nullptr;
 name const * g_eq_intro = nullptr;
@@ -24,6 +26,7 @@ name const * g_eq_symm = nullptr;
 name const * g_eq_trans = nullptr;
 name const * g_exists_elim = nullptr;
 name const * g_false = nullptr;
+name const * g_false_rec = nullptr;
 name const * g_heq = nullptr;
 name const * g_heq_refl = nullptr;
 name const * g_heq_to_eq = nullptr;
@@ -133,6 +136,8 @@ void initialize_constants() {
     g_char = new name{"char"};
     g_char_mk = new name{"char", "mk"};
     g_dite = new name{"dite"};
+    g_empty = new name{"empty"};
+    g_empty_rec = new name{"empty", "rec"};
     g_eq = new name{"eq"};
     g_eq_elim_inv_inv = new name{"eq", "elim_inv_inv"};
     g_eq_intro = new name{"eq", "intro"};
@@ -143,6 +148,7 @@ void initialize_constants() {
     g_eq_trans = new name{"eq", "trans"};
     g_exists_elim = new name{"exists", "elim"};
     g_false = new name{"false"};
+    g_false_rec = new name{"false", "rec"};
     g_heq = new name{"heq"};
     g_heq_refl = new name{"heq", "refl"};
     g_heq_to_eq = new name{"heq", "to_eq"};
@@ -253,6 +259,8 @@ void finalize_constants() {
     delete g_char;
     delete g_char_mk;
     delete g_dite;
+    delete g_empty;
+    delete g_empty_rec;
     delete g_eq;
     delete g_eq_elim_inv_inv;
     delete g_eq_intro;
@@ -263,6 +271,7 @@ void finalize_constants() {
     delete g_eq_trans;
     delete g_exists_elim;
     delete g_false;
+    delete g_false_rec;
     delete g_heq;
     delete g_heq_refl;
     delete g_heq_to_eq;
@@ -372,6 +381,8 @@ name const & get_bool_tt_name() { return *g_bool_tt; }
 name const & get_char_name() { return *g_char; }
 name const & get_char_mk_name() { return *g_char_mk; }
 name const & get_dite_name() { return *g_dite; }
+name const & get_empty_name() { return *g_empty; }
+name const & get_empty_rec_name() { return *g_empty_rec; }
 name const & get_eq_name() { return *g_eq; }
 name const & get_eq_elim_inv_inv_name() { return *g_eq_elim_inv_inv; }
 name const & get_eq_intro_name() { return *g_eq_intro; }
@@ -382,6 +393,7 @@ name const & get_eq_symm_name() { return *g_eq_symm; }
 name const & get_eq_trans_name() { return *g_eq_trans; }
 name const & get_exists_elim_name() { return *g_exists_elim; }
 name const & get_false_name() { return *g_false; }
+name const & get_false_rec_name() { return *g_false_rec; }
 name const & get_heq_name() { return *g_heq; }
 name const & get_heq_refl_name() { return *g_heq_refl; }
 name const & get_heq_to_eq_name() { return *g_heq_to_eq; }
diff --git a/src/library/constants.h b/src/library/constants.h
index bc1c7a964..842998427 100644
--- a/src/library/constants.h
+++ b/src/library/constants.h
@@ -16,6 +16,8 @@ name const & get_bool_tt_name();
 name const & get_char_name();
 name const & get_char_mk_name();
 name const & get_dite_name();
+name const & get_empty_name();
+name const & get_empty_rec_name();
 name const & get_eq_name();
 name const & get_eq_elim_inv_inv_name();
 name const & get_eq_intro_name();
@@ -26,6 +28,7 @@ name const & get_eq_symm_name();
 name const & get_eq_trans_name();
 name const & get_exists_elim_name();
 name const & get_false_name();
+name const & get_false_rec_name();
 name const & get_heq_name();
 name const & get_heq_refl_name();
 name const & get_heq_to_eq_name();
diff --git a/src/library/constants.txt b/src/library/constants.txt
index b5b2266e6..a5f4703e1 100644
--- a/src/library/constants.txt
+++ b/src/library/constants.txt
@@ -9,6 +9,8 @@ bool.tt
 char
 char.mk
 dite
+empty
+empty.rec
 eq
 eq.elim_inv_inv
 eq.intro
@@ -19,6 +21,7 @@ eq.symm
 eq.trans
 exists.elim
 false
+false.rec
 heq
 heq.refl
 heq.to_eq
diff --git a/src/library/tactic/CMakeLists.txt b/src/library/tactic/CMakeLists.txt
index 18a024c23..f87ccfc19 100644
--- a/src/library/tactic/CMakeLists.txt
+++ b/src/library/tactic/CMakeLists.txt
@@ -4,6 +4,6 @@ exact_tactic.cpp generalize_tactic.cpp
 inversion_tactic.cpp whnf_tactic.cpp revert_tactic.cpp
 assert_tactic.cpp clear_tactic.cpp expr_to_tactic.cpp location.cpp
 rewrite_tactic.cpp util.cpp class_instance_synth.cpp init_module.cpp
-change_tactic.cpp check_expr_tactic.cpp let_tactic.cpp)
+change_tactic.cpp check_expr_tactic.cpp let_tactic.cpp contradiction_tactic.cpp)
 
 target_link_libraries(tactic ${LEAN_LIBS})
diff --git a/src/library/tactic/contradiction_tactic.cpp b/src/library/tactic/contradiction_tactic.cpp
new file mode 100644
index 000000000..ae215f067
--- /dev/null
+++ b/src/library/tactic/contradiction_tactic.cpp
@@ -0,0 +1,80 @@
+/*
+Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#include "kernel/inductive/inductive.h"
+#include "library/constants.h"
+#include "library/util.h"
+#include "library/reducible.h"
+#include "library/tactic/intros_tactic.h"
+#include "library/tactic/expr_to_tactic.h"
+#include "kernel/kernel_exception.h"
+
+namespace lean {
+tactic contradiction_tactic() {
+    auto fn = [=](environment const & env, io_state const & ios, proof_state const & s) {
+        goals const & gs = s.get_goals();
+        if (empty(gs)) {
+            throw_no_goal_if_enabled(s);
+            return optional<proof_state>();
+        }
+        goal const & g      = head(gs);
+        expr const & t      = g.get_type();
+        substitution subst  = s.get_subst();
+        name_generator ngen = s.get_ngen();
+        auto tc             = mk_type_checker(env, ngen.mk_child(), s.relax_main_opaque());
+        buffer<expr> hyps;
+        g.get_hyps(hyps);
+        for (expr const & h : hyps) {
+            expr h_type = mlocal_type(h);
+            h_type      = tc->whnf(h_type).first;
+            expr lhs, rhs;
+            if (is_false(env, h_type)) {
+                assign(subst, g, mk_false_rec(*tc, h, t));
+                return some_proof_state(proof_state(s, tail(gs), subst, ngen));
+            } else if (is_eq(h_type, lhs, rhs)) {
+                lhs = tc->whnf(lhs).first;
+                rhs = tc->whnf(rhs).first;
+                optional<name> lhs_c = is_constructor_app(env, lhs);
+                optional<name> rhs_c = is_constructor_app(env, rhs);
+                if (lhs_c && rhs_c && *lhs_c != *rhs_c) {
+                    if (optional<name> I_name = inductive::is_intro_rule(env, *lhs_c)) {
+                        name no_confusion(*I_name, "no_confusion");
+                        try {
+                            expr I      = tc->whnf(tc->infer(lhs).first).first;
+                            buffer<expr> args;
+                            expr I_fn   = get_app_args(I, args);
+                            if (is_constant(I_fn)) {
+                                level t_lvl = sort_level(tc->ensure_type(t).first);
+                                expr V = mk_app(mk_app(mk_constant(no_confusion, cons(t_lvl, const_levels(I_fn))), args),
+                                                t, lhs, rhs, h);
+                                if (auto r = lift_down_if_hott(*tc, V)) {
+                                    tc->check_ignore_undefined_universes(*r);
+                                    assign(subst, g, *r);
+                                    return some_proof_state(proof_state(s, tail(gs), subst, ngen));
+                                }
+                            }
+                        } catch (kernel_exception & ex) {
+                            regular(env, ios) << ex << "\n";
+                        }
+                    }
+                }
+            }
+        }
+        return none_proof_state();
+    };
+    return tactic01(fn);
+}
+
+void initialize_contradiction_tactic() {
+    register_tac(name{"tactic", "contradiction"},
+                 [](type_checker &, elaborate_fn const &, expr const &, pos_info_provider const *) {
+                     list<name> empty;
+                     return then(orelse(intros_tactic(empty), id_tactic()), contradiction_tactic());
+                 });
+}
+void finalize_contradiction_tactic() {
+}
+}
diff --git a/src/library/tactic/contradiction_tactic.h b/src/library/tactic/contradiction_tactic.h
new file mode 100644
index 000000000..c21c85cb7
--- /dev/null
+++ b/src/library/tactic/contradiction_tactic.h
@@ -0,0 +1,11 @@
+/*
+Copyright (c) 2015 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#pragma once
+namespace lean {
+void initialize_contradiction_tactic();
+void finalize_contradiction_tactic();
+}
diff --git a/src/library/tactic/init_module.cpp b/src/library/tactic/init_module.cpp
index b725f1484..0a47ce937 100644
--- a/src/library/tactic/init_module.cpp
+++ b/src/library/tactic/init_module.cpp
@@ -23,6 +23,7 @@ Author: Leonardo de Moura
 #include "library/tactic/change_tactic.h"
 #include "library/tactic/check_expr_tactic.h"
 #include "library/tactic/let_tactic.h"
+#include "library/tactic/contradiction_tactic.h"
 
 namespace lean {
 void initialize_tactic_module() {
@@ -45,9 +46,11 @@ void initialize_tactic_module() {
     initialize_change_tactic();
     initialize_check_expr_tactic();
     initialize_let_tactic();
+    initialize_contradiction_tactic();
 }
 
 void finalize_tactic_module() {
+    finalize_contradiction_tactic();
     finalize_let_tactic();
     finalize_check_expr_tactic();
     finalize_change_tactic();
diff --git a/src/library/tactic/intros_tactic.cpp b/src/library/tactic/intros_tactic.cpp
index 3cd0969e6..d09c97fef 100644
--- a/src/library/tactic/intros_tactic.cpp
+++ b/src/library/tactic/intros_tactic.cpp
@@ -11,7 +11,7 @@ Author: Leonardo de Moura
 #include "library/tactic/expr_to_tactic.h"
 
 namespace lean {
-tactic intros_tactic(list<name> _ns, bool relax_main_opaque) {
+tactic intros_tactic(list<name> _ns) {
     auto fn = [=](environment const & env, io_state const &, proof_state const & s) {
         list<name> ns    = _ns;
         goals const & gs = s.get_goals();
@@ -21,7 +21,7 @@ tactic intros_tactic(list<name> _ns, bool relax_main_opaque) {
         }
         goal const & g      = head(gs);
         name_generator ngen = s.get_ngen();
-        auto tc             = mk_type_checker(env, ngen.mk_child(), relax_main_opaque);
+        auto tc             = mk_type_checker(env, ngen.mk_child(), s.relax_main_opaque());
         expr t              = g.get_type();
         expr m              = g.get_meta();
         bool gen_names      = empty(ns);
diff --git a/src/library/tactic/intros_tactic.h b/src/library/tactic/intros_tactic.h
index 16b1bbcb5..83d5da17b 100644
--- a/src/library/tactic/intros_tactic.h
+++ b/src/library/tactic/intros_tactic.h
@@ -7,7 +7,7 @@ Author: Leonardo de Moura
 #pragma once
 #include "library/tactic/tactic.h"
 namespace lean {
-tactic intros_tactic(list<name> ns, bool relax_main_opaque = true);
+tactic intros_tactic(list<name> ns);
 void initialize_intros_tactic();
 void finalize_intros_tactic();
 }
diff --git a/src/library/tactic/inversion_tactic.cpp b/src/library/tactic/inversion_tactic.cpp
index a2cd0629d..b9dd010d3 100644
--- a/src/library/tactic/inversion_tactic.cpp
+++ b/src/library/tactic/inversion_tactic.cpp
@@ -507,7 +507,7 @@ class inversion_tac {
             throw inversion_exception();
     }
 
-    void throw_lift_down_failure() {
+    [[ noreturn ]] void throw_lift_down_failure() {
         if (m_throw_tactic_exception)
             throw tactic_exception("invalid 'cases' tactic, lift.down failed");
         else
@@ -664,19 +664,10 @@ class inversion_tac {
        We must apply lift.down to eliminate the auxiliary lift.
     */
     expr lift_down(expr const & v) {
-        if (!m_proof_irrel) {
-            expr v_type       = whnf(infer_type(v));
-            if (!is_app(v_type)) {
-                throw_lift_down_failure();
-            }
-            expr const & lift = app_fn(v_type);
-            if (!is_constant(lift) || const_name(lift) != get_lift_name()) {
-                throw_lift_down_failure();
-            }
-            return mk_app(mk_constant(get_lift_down_name(), const_levels(lift)), app_arg(v_type), v);
-        } else {
-            return v;
-        }
+        if (auto r = lift_down_if_hott(m_tc, v))
+            return *r;
+        else
+            throw_lift_down_failure();
     }
 
     buffer<expr>        m_c_args; // current intro/constructor args that may be renamed by unify_eqs
diff --git a/src/library/util.cpp b/src/library/util.cpp
index 66520dea4..f46939eba 100644
--- a/src/library/util.cpp
+++ b/src/library/util.cpp
@@ -15,6 +15,10 @@ Author: Leonardo de Moura
 #include "library/constants.h"
 
 namespace lean {
+bool is_standard(environment const & env) {
+    return env.prop_proof_irrel() && env.impredicative();
+}
+
 bool is_def_app(environment const & env, expr const & e) {
     if (!is_app(e))
         return false;
@@ -151,8 +155,8 @@ level get_datatype_level(expr ind_type) {
 }
 
 bool is_inductive_predicate(environment const & env, name const & n) {
-    if (!env.impredicative())
-        return false; // environment does not have Prop
+    if (!is_standard(env))
+        return false;
     if (!inductive::is_inductive_decl(env, n))
         return false; // n is not inductive datatype
     return is_zero(get_datatype_level(env.get(n).get_type()));
@@ -240,6 +244,43 @@ expr to_telescope(type_checker & tc, expr type, buffer<expr> & telescope, option
     return to_telescope(tc, type, telescope, binfo, cs);
 }
 
+bool is_false(expr const & e) {
+    return is_constant(e) && const_name(e) == get_false_name();
+}
+
+bool is_empty(expr const & e) {
+    return is_constant(e) && const_name(e) == get_empty_name();
+}
+
+bool is_false(environment const & env, expr const & e) {
+    return is_standard(env) ? is_false(e) : is_empty(e);
+}
+
+expr mk_false_rec(type_checker & tc, expr const & f, expr const & t) {
+    level t_lvl = sort_level(tc.ensure_type(t).first);
+    if (is_standard(tc.env())) {
+        return mk_app(mk_constant(get_false_rec_name(), {t_lvl}), t, f);
+    } else {
+        expr f_type = tc.infer(f).first;
+        level f_lvl = sort_level(tc.ensure_type(f_type).first);
+        return mk_app(mk_constant(get_empty_rec_name(), {t_lvl, f_lvl}), mk_lambda("e", f_type, t), f);
+    }
+}
+
+optional<expr> lift_down_if_hott(type_checker & tc, expr const & v) {
+    if (is_standard(tc.env())) {
+        return some_expr(v);
+    } else {
+        expr v_type = tc.whnf(tc.infer(v).first).first;
+        if (!is_app(v_type))
+            return none_expr();
+        expr const & lift = app_fn(v_type);
+        if (!is_constant(lift) || const_name(lift) != get_lift_name())
+            return none_expr();
+        return some_expr(mk_app(mk_constant(get_lift_down_name(), const_levels(lift)), app_arg(v_type), v));
+    }
+}
+
 static expr * g_true = nullptr;
 static expr * g_true_intro = nullptr;
 static expr * g_and = nullptr;
@@ -387,6 +428,14 @@ bool is_eq(expr const & e) {
     return is_constant(fn) && const_name(fn) == get_eq_name();
 }
 
+bool is_eq(expr const & e, expr & lhs, expr & rhs) {
+    if (!is_eq(e) || !is_app(app_fn(e)))
+        return false;
+    lhs = app_arg(app_fn(e));
+    rhs = app_arg(e);
+    return true;
+}
+
 bool is_eq_a_a(expr const & e) {
     if (!is_eq(e))
         return false;
diff --git a/src/library/util.h b/src/library/util.h
index ad97df721..e3e2c370c 100644
--- a/src/library/util.h
+++ b/src/library/util.h
@@ -26,6 +26,9 @@ optional<expr> unfold_app(environment const & env, expr const & e);
 */
 optional<level> dec_level(level const & l);
 
+/** \brief Return true iff \c env has been configured with an impredicative and proof irrelevant Prop. */
+bool is_standard(environment const & env);
+
 bool has_unit_decls(environment const & env);
 bool has_eq_decls(environment const & env);
 bool has_heq_decls(environment const & env);
@@ -123,11 +126,20 @@ expr mk_pair(type_checker & tc, expr const & a, expr const & b, bool prop);
 expr mk_pr1(type_checker & tc, expr const & p, bool prop);
 expr mk_pr2(type_checker & tc, expr const & p, bool prop);
 
+
+bool is_false(expr const & e);
+bool is_empty(expr const & e);
+/** \brief Return true iff \c e is false (in standard mode) or empty (in HoTT) mode */
+bool is_false(environment const & env, expr const & e);
+/** \brief Return an element of type t given an element \c f : false (in standard mode) and empty (in HoTT) mode */
+expr mk_false_rec(type_checker & tc, expr const & f, expr const & t);
+
 expr mk_eq(type_checker & tc, expr const & lhs, expr const & rhs);
 expr mk_refl(type_checker & tc, expr const & a);
 expr mk_symm(type_checker & tc, expr const & H);
 bool is_eq_rec(expr const & e);
 bool is_eq(expr const & e);
+bool is_eq(expr const & e, expr & lhs, expr & rhs);
 /** \brief Return true iff \c e is of the form (eq A a a) */
 bool is_eq_a_a(expr const & e);
 /** \brief Return true iff \c e is of the form (eq A a a') where \c a and \c a' are definitionally equal */
@@ -137,6 +149,12 @@ bool is_iff(expr const & e);
 expr mk_iff(expr const & lhs, expr const & rhs);
 expr mk_iff_refl(expr const & a);
 
+/** \brief If in HoTT mode, apply lift.down.
+    The no_confusion constructions uses lifts in the proof relevant version (aka HoTT mode).
+    We must apply lift.down to eliminate the auxiliary lift.
+*/
+optional<expr> lift_down_if_hott(type_checker & tc, expr const & v);
+
 /** \brief Create a telescope equality for HoTT library.
     This procedure assumes eq supports dependent elimination.
     For HoTT, we can't use heterogeneous equality.
diff --git a/tests/lean/hott/contra1.hlean b/tests/lean/hott/contra1.hlean
new file mode 100644
index 000000000..b39aa9a6c
--- /dev/null
+++ b/tests/lean/hott/contra1.hlean
@@ -0,0 +1,16 @@
+example (a b : nat) (h : empty) : a = b :=
+by contradiction
+
+example : ∀ (a b : nat), empty → a = b :=
+by contradiction
+
+example : ∀ (a b : nat), 0 = 1 → a = b :=
+by contradiction
+
+definition id {A : Type} (a : A) := a
+
+example : ∀ (a b : nat), id empty → a = b :=
+by contradiction
+
+example : ∀ (a b : nat), id (0 = 1) → a = b :=
+by contradiction
diff --git a/tests/lean/run/contra1.lean b/tests/lean/run/contra1.lean
new file mode 100644
index 000000000..cd22b35f5
--- /dev/null
+++ b/tests/lean/run/contra1.lean
@@ -0,0 +1,16 @@
+example (a b : nat) (h : false) : a = b :=
+by contradiction
+
+example : ∀ (a b : nat), false → a = b :=
+by contradiction
+
+example : ∀ (a b : nat), 0 = 1 → a = b :=
+by contradiction
+
+definition id {A : Type} (a : A) := a
+
+example : ∀ (a b : nat), id false → a = b :=
+by contradiction
+
+example : ∀ (a b : nat), id (0 = 1) → a = b :=
+by contradiction