diff --git a/Cslib/Foundations/Logic/Connectives.lean b/Cslib/Foundations/Logic/Connectives.lean
new file mode 100644
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+++ b/Cslib/Foundations/Logic/Connectives.lean
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+/-
+Copyright (c) 2025 Thomas Waring. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Thomas Waring
+-/
+
+module
+
+public import Cslib.Init
+public import Mathlib.Order.Notation
+
+/-! # Notation classes for logical connectives -/
+
+@[expose] public section
+
+namespace Cslib.Logic
+
+/-- Class for implication. -/
+class HasImpl (α : Type*) where
+  /-- Implication. -/
+  impl : α → α → α
+
+@[inherit_doc] scoped infixr:25 " → " => HasImpl.impl
+
+/-- Class for conjunction. -/
+class HasAnd (α : Type*) where
+  /-- Conjunction. -/
+  and : α → α → α
+
+@[inherit_doc] scoped infixr:35 " ∧ " => HasAnd.and
+
+/-- Class for disjunction. -/
+class HasOr (α : Type*) where
+  /-- Disjunction. -/
+  or : α → α → α
+
+@[inherit_doc] scoped infixr:30 " ∨ " => HasOr.or
+
+/-- Class for negation. -/
+class HasNot (α : Type*) where
+  /-- Negation. -/
+  not : α → α
+
+@[inherit_doc] scoped notation:max "¬" a:40 => HasNot.not a
+
+/-- Instantiate negation for types with implication and a bottom element. NB: this has a lower
+  instance priority to account for proposition types with inbuilt negation. Possibly it should be
+  a `def` instead? -/
+instance (priority := 900) instNotImplBot (α : Type*) [HasImpl α] [Bot α] : HasNot α where
+  not a := a → ⊥
+
+end Cslib.Logic
diff --git a/Cslib/Foundations/Logic/Model.lean b/Cslib/Foundations/Logic/Model.lean
new file mode 100644
index 000000000..49c9a091d
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+++ b/Cslib/Foundations/Logic/Model.lean
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+/-
+Copyright (c) 2025 Thomas Waring. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Thomas Waring
+-/
+
+module
+
+public import Cslib.Foundations.Logic.Connectives
+public import Cslib.Foundations.Logic.InferenceSystem
+public import Cslib.Logics.Modal.Basic
+public import Cslib.Logics.Propositional.NaturalDeduction.Basic
+
+/-! # Semantics for logical systems -/
+
+public section
+
+namespace Cslib.Logic
+
+/-- Objects of type `β` carry a forcing relation with the proposition type `α`. -/
+class ModelClass (α : outParam Type*) (β : Type*) where
+  /-- `Forces b a` has the intended semantics "`a` is valid in the model `b`". -/
+  Forces : β → α → Prop
+
+scoped notation "⊨[" b "] " a => ModelClass.Forces b a
+
+/-- Objects of type `β` carry a forcing relation worlds of type `γ` and the proposition type `α`. -/
+class ParamModelClass (α : outParam Type*) (β : Type*) where
+  Param : β → Type*
+  /-- Forcing relation associated to each parameter. -/
+  ForcesAt (b : β) : (Param b) → α → Prop
+
+scoped notation w " ⊨[" b "] " a => ParamModelClass.ForcesAt b w a
+
+instance ParamModelClass.toModelClass {α β : Type*} [ParamModelClass α β] : ModelClass α β where
+  Forces M A := ∀ w : ParamModelClass.Param M, w ⊨[M] A
+
+/-- Bundled interpretation function. -/
+class HasInterp (α : outParam Type*) (β : Type*) where
+  /-- Type carrying interpretation. -/
+  Ground : β → Type*
+  /-- Interpret a proposition in a model. -/
+  interp : (b : β) → α → Ground b
+
+/-- An `InterpModel` consists of an interpretation function, and a set specifying which
+  interpretations are considered valid. -/
+class InterpModelClass (α : outParam (Type*)) (β : Type*) extends HasInterp α β where
+  /-- The set of valid interpretations. -/
+  filter (b : β) : Set (Ground b)
+
+instance InterpModelClass.instModelClass {α β : Type*} [InterpModelClass α β] : ModelClass α β where
+  Forces b a := HasInterp.interp b a ∈ filter b
+
+namespace HasInterp
+
+class AlgebraicAnd (α β : Type*) [HasInterp α β] [HasAnd α] [∀ b : β, Min (Ground b)] where
+  interp_and_eq (M : β) (x y : α) : interp M (x ∧ y) = interp M x ⊓ interp M y
+
+class AlgebraicOr (α β : Type*) [HasInterp α β] [HasOr α] [∀ b : β, Max (Ground b)] where
+  interp_or_eq (M : β) (x y : α) : interp M (x ∨ y) = interp M x ⊔ interp M y
+
+class AlgebraicImpl (α β : Type*) [HasInterp α β] [HasImpl α] [∀ b : β, HImp (Ground b)] where
+  interp_impl_eq (M : β) (x y : α) : interp M (x → y) = interp M x ⇨ interp M y
+
+class AlgebraicNot (α β : Type*) [HasInterp α β] [HasNot α] [∀ b : β, Compl (Ground b)] where
+  interp_not_eq (M : β) (x y : α) : interp M (¬ x) = (interp M x)ᶜ
+
+end HasInterp
+
+open ModelClass ParamModelClass InferenceSystem
+
+variable {α β T} [ModelClass α β] [InferenceSystem T α]
+
+def SoundFor (α β T) [ModelClass α β] [InferenceSystem T α] (S : Set β) : Prop :=
+  ∀ (A : α), DerivableIn T A → ∀ M ∈ S, ⊨[M] A
+
+lemma SoundFor.subset {S S' : Set β} (hS : S ⊆ S') : SoundFor α β T S' → SoundFor α β T S :=
+  fun h A hA M hM => h A hA M (hS hM)
+
+def CompleteFor (α β T : Type*) [ModelClass α β] [InferenceSystem T α] (S : Set β) : Prop :=
+  ∀ A : α, (∀ M ∈ S, ⊨[M] A) → DerivableIn T A
+
+lemma CompleteFor.supset {S S' : Set β} (hS : S ⊆ S') :
+    CompleteFor α β T S → CompleteFor α β T S' := fun h A hA => h A (fun M hM => hA M (hS hM))
+
+def IsCompleteModel (α β T) [ModelClass α β] [InferenceSystem T α] (M : β) : Prop :=
+  ∀ (A : α), (⊨[M] A) → DerivableIn T A
+
+def ParamModelClass.theory {α β : Type*} [ParamModelClass α β] {M : β} (w : Param M) : Set α :=
+  {A : α | w ⊨[M] A}
+
+def ModelClass.logic {α β : Type*} [ModelClass α β] (S : Set β) : Set α := {A | ∀ b ∈ S, ⊨[b] A}
+
+namespace Modal
+
+structure BundledModel (Atom : Type*) where
+  World : Type*
+  model : Model World Atom
+
+def Model.toBundledModel {World Atom : Type*} (M : Model World Atom) : BundledModel Atom :=
+  {World := World, model := M}
+
+instance {Atom : Type*} : ParamModelClass (Proposition Atom) (BundledModel Atom) where
+  Param M := M.World
+  ForcesAt M w A := Satisfies M.model w A
+
+example {World Atom : Type*} (S : Set (Model World Atom)) :
+    logic S = ModelClass.logic (Model.toBundledModel '' S) := by
+  simp [ModelClass.logic]
+  rfl
+
+example {World Atom : Type*} (m : Model World Atom) (w : World) :
+    theory m w = ParamModelClass.theory (M := m.toBundledModel) w := by
+  simp [theory, ParamModelClass.theory]
+  rfl
+
+end Modal
+
+namespace PL
+
+variable {Atom : Type*}
+
+instance : HasAnd (Proposition Atom) := ⟨Proposition.and⟩
+instance : HasOr (Proposition Atom) := ⟨Proposition.or⟩
+instance : HasImpl (Proposition Atom) := ⟨Proposition.impl⟩
+instance [Bot Atom] : HasNot (Proposition Atom) := ⟨Proposition.neg⟩
+
+structure HeytingModel (Atom : Type*) where
+  H : Type*
+  [inst : GeneralizedHeytingAlgebra H]
+  v : Atom → H
+
+instance (M : HeytingModel Atom) : GeneralizedHeytingAlgebra M.H := M.inst
+
+def HeytingModel.interp (M : HeytingModel Atom) : Proposition Atom → M.H
+  | Proposition.atom x => M.v x
+  | Proposition.and A B => M.interp A ⊓ M.interp B
+  | Proposition.or A B => M.interp A ⊔ M.interp B
+  | Proposition.impl A B => M.interp A ⇨ M.interp B
+
+instance : InterpModelClass (Proposition Atom) (HeytingModel Atom) where
+  Ground M := M.H
+  interp := HeytingModel.interp
+  filter _ := {⊤}
+
+instance (M : HeytingModel Atom) : GeneralizedHeytingAlgebra (HasInterp.Ground M) := M.inst
+
+instance : HasInterp.AlgebraicAnd (Proposition Atom) (HeytingModel Atom) where
+  interp_and_eq _ _ _ := rfl
+
+instance : HasInterp.AlgebraicOr (Proposition Atom) (HeytingModel Atom) where
+  interp_or_eq _ _ _ := rfl
+
+instance : HasInterp.AlgebraicImpl (Proposition Atom) (HeytingModel Atom) where
+  interp_impl_eq _ _ _ := rfl
+
+theorem HeytingModel.sound [DecidableEq Atom] {T : Theory Atom} :
+    SoundFor (Proposition Atom) (HeytingModel Atom) T {M | ∀ A ∈ T, interp M A = ⊤} :=
+  sorry -- i have this in a branch
+
+def Theory.lindenbaum [DecidableEq Atom] (T : Theory Atom) : HeytingModel Atom :=
+  sorry -- usual Heyting-algebra of propositions modulo equivalence
+
+theorem Theory.lindenbaum_complete [DecidableEq Atom] {T : Theory Atom} :
+    IsCompleteModel (Proposition Atom) (HeytingModel Atom) T T.lindenbaum :=
+  sorry -- also in a branch
+
+end PL
+
+end Cslib.Logic