diff --git a/crates/fj-core/src/layers/layer.rs b/crates/fj-core/src/layers/layer.rs
index efa1efcbe..4f4148a9a 100644
--- a/crates/fj-core/src/layers/layer.rs
+++ b/crates/fj-core/src/layers/layer.rs
@@ -10,17 +10,14 @@ use std::ops::Deref;
 /// processed by [`Layer::process`]. Processing a command can result in any
 /// number of events, which can then be used as commands for other layers.
 ///
-/// All of this is mediated through [`State`], which the wrapped state must
-/// implement.
-///
 /// This design takes inspiration from, and uses the nomenclature of, this
 /// article:
 /// <https://thinkbeforecoding.com/post/2021/12/17/functional-event-sourcing-decider>
-pub struct Layer<S: State> {
+pub struct Layer<S> {
     state: S,
 }
 
-impl<S: State> Layer<S> {
+impl<S> Layer<S> {
     /// Create an instance of `Layer`
     pub fn new(state: S) -> Self {
         Self { state }
@@ -30,11 +27,14 @@ impl<S: State> Layer<S> {
     ///
     /// The command is processed synchronously. When this method returns, the
     /// state has been updated.
-    pub fn process(&mut self, command: S::Command, events: &mut Vec<S::Event>) {
-        self.state.decide(command, events);
+    pub fn process<C>(&mut self, command: C, events: &mut Vec<C::Event>)
+    where
+        C: Command<S>,
+    {
+        command.decide(&self.state, events);
 
         for event in events {
-            self.state.evolve(event);
+            event.evolve(&mut self.state);
         }
     }
 
@@ -44,7 +44,7 @@ impl<S: State> Layer<S> {
     }
 }
 
-impl<S: State> Deref for Layer<S> {
+impl<S> Deref for Layer<S> {
     type Target = S;
 
     fn deref(&self) -> &Self::Target {
@@ -52,7 +52,7 @@ impl<S: State> Deref for Layer<S> {
     }
 }
 
-impl<S: State> Default for Layer<S>
+impl<S> Default for Layer<S>
 where
     S: Default,
 {
@@ -61,37 +61,30 @@ where
     }
 }
 
-/// The state of a specific layer
-///
-/// Implementations of this trait are wrapped by the generic [`Layer`], which is
-/// the consumer of this trait's API.
-///
-/// See [`Layer`] for a more detailed explanation.
-pub trait State {
-    /// A command that encodes a request to update the state
-    ///
-    /// Commands are processed by [`State::decide`].
-    type Command;
-
+/// A command that encodes a request to update a layer's state
+pub trait Command<S> {
     /// An event that encodes a change to the state
     ///
-    /// Events are produced by [`State::decide`] and processed by
-    /// [`State::evolve`].
-    type Event;
+    /// Events are produced by [`Command::decide`] and processed by
+    /// [`Event::evolve`].
+    type Event: Event<S>;
 
-    /// Decide how to react to the provided command
+    /// Decide which events to produce, given the command and provided state
     ///
     /// If the command must result in changes to the state, any number of events
     /// that describe these state changes can be produced.
-    fn decide(&self, command: Self::Command, events: &mut Vec<Self::Event>);
+    fn decide(self, state: &S, events: &mut Vec<Self::Event>);
+}
 
-    /// Evolve the state according to the provided event
+/// An event that encodes a change to a layer's state
+pub trait Event<S> {
+    /// Evolve the provided state
     ///
-    /// This is the only method that gets mutable access to the state, making
-    /// sure that all changes to the state are captured as events.
+    /// This is the only method that [`Layer`] gives mutable access to the
+    /// state, making sure that all changes to the state are captured as events.
     ///
     /// Implementations of this method are supposed to be relatively dumb. Any
     /// decisions that go into updating the state should be made in
-    /// [`State::decide`], and encoded into the event.
-    fn evolve(&mut self, event: &Self::Event);
+    /// [`Command::decide`], and encoded into the event.
+    fn evolve(&self, state: &mut S);
 }
diff --git a/crates/fj-core/src/layers/layers.rs b/crates/fj-core/src/layers/layers.rs
index 2682537a8..200fd4529 100644
--- a/crates/fj-core/src/layers/layers.rs
+++ b/crates/fj-core/src/layers/layers.rs
@@ -21,7 +21,7 @@ use super::Layer;
 /// That can be changed, once necessary.
 #[derive(Default)]
 pub struct Layers {
-    /// The objects layers
+    /// The objects layer
     ///
     /// Manages the stores of topological and geometric objects that make up
     /// shapes.
diff --git a/crates/fj-core/src/layers/mod.rs b/crates/fj-core/src/layers/mod.rs
index 953ff94f3..d18457b73 100644
--- a/crates/fj-core/src/layers/mod.rs
+++ b/crates/fj-core/src/layers/mod.rs
@@ -9,6 +9,6 @@ mod layer;
 mod layers;
 
 pub use self::{
-    layer::{Layer, State},
+    layer::{Command, Event, Layer},
     layers::Layers,
 };
diff --git a/crates/fj-core/src/layers/objects.rs b/crates/fj-core/src/layers/objects.rs
index 4d6828ab4..b5cff0ea2 100644
--- a/crates/fj-core/src/layers/objects.rs
+++ b/crates/fj-core/src/layers/objects.rs
@@ -5,58 +5,46 @@ use crate::{
     validate::Validation,
 };
 
-use super::{Layer, State};
+use super::{Command, Event, Layer};
 
 impl Layer<Objects> {
-    /// Insert and object into the stores
+    /// Insert an object into the stores
+    ///
+    /// Passes any events produced to the validation layer.
     pub fn insert(
         &mut self,
         object: AnyObject<AboutToBeStored>,
         validation: &mut Layer<Validation>,
     ) {
         let mut events = Vec::new();
-        self.process(ObjectsCommand::InsertObject { object }, &mut events);
+        self.process(InsertObject { object }, &mut events);
 
         for event in events {
-            validation.on_objects_event(event);
+            validation.process(event, &mut Vec::new());
         }
     }
 }
 
-impl State for Objects {
-    type Command = ObjectsCommand;
-    type Event = ObjectsEvent;
+/// Insert an object into the stores
+///
+/// This struct serves as both event and command for `Layer<Objects>`, as well
+/// as a command for `Layer<Validation>`.
+#[derive(Clone, Debug)]
+pub struct InsertObject {
+    /// The object to insert
+    pub object: AnyObject<AboutToBeStored>,
+}
 
-    fn decide(&self, command: Self::Command, events: &mut Vec<Self::Event>) {
-        let ObjectsCommand::InsertObject { object } = command;
-        events.push(ObjectsEvent::InsertObject { object });
-    }
+impl Command<Objects> for InsertObject {
+    type Event = InsertObject;
 
-    fn evolve(&mut self, event: &Self::Event) {
-        let ObjectsEvent::InsertObject { object } = event;
-        object.clone().insert(self);
+    fn decide(self, _: &Objects, events: &mut Vec<Self::Event>) {
+        events.push(self);
     }
 }
 
-/// Command for `Layer<Objects>`
-#[derive(Debug)]
-pub enum ObjectsCommand {
-    /// Insert an object into the stores
-    ///
-    /// This is the one primitive operation that all other operations are built
-    /// upon.
-    InsertObject {
-        /// The object to insert
-        object: AnyObject<AboutToBeStored>,
-    },
-}
-
-/// Event produced by `Layer<Objects>`
-#[derive(Clone, Debug)]
-pub enum ObjectsEvent {
-    /// Insert an object into the stores
-    InsertObject {
-        /// The object to insert
-        object: AnyObject<AboutToBeStored>,
-    },
+impl Event<Objects> for InsertObject {
+    fn evolve(&self, state: &mut Objects) {
+        self.object.clone().insert(state);
+    }
 }
diff --git a/crates/fj-core/src/layers/validation.rs b/crates/fj-core/src/layers/validation.rs
index b826045e4..96071ee9e 100644
--- a/crates/fj-core/src/layers/validation.rs
+++ b/crates/fj-core/src/layers/validation.rs
@@ -5,18 +5,9 @@ use crate::{
     validate::{Validation, ValidationError, ValidationErrors},
 };
 
-use super::{objects::ObjectsEvent, Layer, State};
+use super::{objects::InsertObject, Command, Event, Layer};
 
 impl Layer<Validation> {
-    /// Handler for [`ObjectsEvent`]
-    pub fn on_objects_event(&mut self, event: ObjectsEvent) {
-        let ObjectsEvent::InsertObject { object } = event;
-        let command = ValidationCommand::ValidateObject {
-            object: object.into(),
-        };
-        self.process(command, &mut Vec::new());
-    }
-
     /// Consume the validation layer, returning any validation errors
     pub fn into_result(self) -> Result<(), ValidationErrors> {
         let errors = self.into_state().into_errors();
@@ -29,54 +20,38 @@ impl Layer<Validation> {
     }
 }
 
-impl State for Validation {
-    type Command = ValidationCommand;
-    type Event = ValidationEvent;
+impl Command<Validation> for InsertObject {
+    type Event = ValidationFailed;
 
-    fn decide(&self, command: Self::Command, events: &mut Vec<Self::Event>) {
+    fn decide(self, state: &Validation, events: &mut Vec<Self::Event>) {
         let mut errors = Vec::new();
 
-        match command {
-            ValidationCommand::ValidateObject { object } => {
-                object.validate_with_config(&self.config, &mut errors);
-
-                for err in errors {
-                    events.push(ValidationEvent::ValidationFailed {
-                        object: object.clone(),
-                        err,
-                    });
-                }
-            }
-        }
-    }
+        let object: AnyObject<Stored> = self.object.into();
+        object.validate_with_config(&state.config, &mut errors);
 
-    fn evolve(&mut self, event: &Self::Event) {
-        match event {
-            ValidationEvent::ValidationFailed { object, err } => {
-                self.errors.insert(object.id(), err.clone());
-            }
+        for err in errors {
+            events.push(ValidationFailed {
+                object: object.clone(),
+                err,
+            });
         }
     }
 }
 
-/// Command for `Layer<Validation>`
-pub enum ValidationCommand {
-    /// Validate the provided object
-    ValidateObject {
-        /// The object to validate
-        object: AnyObject<Stored>,
-    },
-}
-
-/// Event produced by `Layer<Validation>`
+/// Validation of an object failed
+///
+/// Event produced by `Layer<Validation>`.
 #[derive(Clone)]
-pub enum ValidationEvent {
-    /// Validation of an object failed
-    ValidationFailed {
-        /// The object for which validation failed
-        object: AnyObject<Stored>,
+pub struct ValidationFailed {
+    /// The object for which validation failed
+    pub object: AnyObject<Stored>,
 
-        /// The validation error
-        err: ValidationError,
-    },
+    /// The validation error
+    pub err: ValidationError,
+}
+
+impl Event<Validation> for ValidationFailed {
+    fn evolve(&self, state: &mut Validation) {
+        state.errors.insert(self.object.id(), self.err.clone());
+    }
 }