diff --git a/Statlib.lean b/Statlib.lean
index 2f08ecf..707dba3 100644
--- a/Statlib.lean
+++ b/Statlib.lean
@@ -1,3 +1,4 @@
+import Statlib.Causal.PotentialResponse
 import Statlib.EValues.DPI
 import Statlib.EValues.EVariable
 import Statlib.EValues.Utility.Basic
diff --git a/Statlib/Causal/PotentialResponse.lean b/Statlib/Causal/PotentialResponse.lean
new file mode 100644
index 0000000..d92bb3f
--- /dev/null
+++ b/Statlib/Causal/PotentialResponse.lean
@@ -0,0 +1,100 @@
+/-
+Copyright (c) 2026 Bo Cowgill. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Bo Cowgill
+-/
+
+module
+
+public import Mathlib.Init
+
+/-!
+# Potential Responses
+
+This module supplies an assumption-free functional representation of potential responses.
+
+`PotentialResponse.id` is the identity for same-unit substitution.
+`PotentialResponse.select` produces a theoretical selected response, not a recorded outcome.
+`PotentialResponse.comp` performs same-unit substitution.
+
+For example, given `mediator : PotentialResponse Treatment Unit Mediator` and
+`outcome : PotentialResponse (Treatment × Mediator) Unit Outcome`, the response
+
+```lean
+outcome.comp (fun (treatment, mediatorTreatment) unit ↦
+  (treatment, mediator mediatorTreatment unit))
+```
+
+evaluated at `(a, a')` and `u` is `outcome (a, mediator a' u) u`, representing the nested
+response $Y(a, M(a'))$. The mediator and outcome responses are evaluated at the same unit `u`.
+
+Causal assumptions, estimands, estimators, probability structure, and statistical inference are
+deferred to later modules.
+-/
+
+@[expose] public section
+
+/-- A potential response assigns a value to each intervention and unit. -/
+abbrev PotentialResponse
+    (Intervention : Type*) (Unit : Type*) (Value : Type*) :=
+  Intervention → Unit → Value
+
+namespace PotentialResponse
+
+variable {Intervention : Type*} {Unit : Type*} {Value : Type*}
+  {Index : Type*} {Source : Type*}
+
+/-- The identity potential response returns its intervention and ignores the unit. -/
+def id : PotentialResponse Intervention Unit Intervention :=
+  fun intervention _ ↦ intervention
+
+/-- Selects each unit's response at the intervention supplied for that unit. -/
+def select
+    (response : PotentialResponse Intervention Unit Value)
+    (intervention : Unit → Intervention) :
+    Unit → Value :=
+  fun u ↦ response (intervention u) u
+
+/-- Substitutes an intervention response into a response, evaluating both at the same unit. -/
+def comp
+    (response : PotentialResponse Intervention Unit Value)
+    (intervention :
+      PotentialResponse Index Unit Intervention) :
+    PotentialResponse Index Unit Value :=
+  fun index u ↦ response (intervention index u) u
+
+@[simp] theorem id_apply (intervention : Intervention) (u : Unit) :
+    (id : PotentialResponse Intervention Unit Intervention) intervention u = intervention := rfl
+
+@[simp] theorem select_apply
+    (response : PotentialResponse Intervention Unit Value)
+    (intervention : Unit → Intervention)
+    (u : Unit) :
+    response.select intervention u =
+      response (intervention u) u := rfl
+
+@[simp] theorem comp_apply
+    (response : PotentialResponse Intervention Unit Value)
+    (intervention :
+      PotentialResponse Index Unit Intervention)
+    (index : Index)
+    (u : Unit) :
+    response.comp intervention index u =
+      response (intervention index u) u := rfl
+
+@[simp] theorem comp_id
+    (response : PotentialResponse Intervention Unit Value) :
+    response.comp id = response := rfl
+
+@[simp] theorem id_comp
+    (response : PotentialResponse Intervention Unit Value) :
+    id.comp response = response := rfl
+
+theorem comp_assoc
+    (response : PotentialResponse Intervention Unit Value)
+    (intervention : PotentialResponse Index Unit Intervention)
+    (source : PotentialResponse Source Unit Index) :
+    (response.comp intervention).comp source =
+      response.comp (intervention.comp source) := rfl
+
+end PotentialResponse