diff --git a/src/engine/Game.ts b/src/engine/Game.ts
index eb68244..03a4a2e 100644
--- a/src/engine/Game.ts
+++ b/src/engine/Game.ts
@@ -1,12 +1,19 @@
 import {enableMapSet, createDraft, finishDraft, Draft} from 'immer'
-import GameState, { Room, Door, Item, ObjectType } from './types/GameState'
+import GameState, { GameObject, Room, Door, Item, ObjectType } from './types/GameState'
+import ParsedCommand, { TokenType, ParsedTokenExpression, ValidCommandDetails, InvalidCommandDetails } from './types/ParsedCommand'
 
 enableMapSet()
 
 type ChangeListener = (state : GameState) => void
 
+
+export type CommandValidateResult = {
+  validCommands: ValidCommandDetails [],
+  invalidCommands: InvalidCommandDetails []
+}
+
 export default class Game {
-  private gameState : GameState = {directions: new Map(), rooms: new Map(), doors: new Map(), items: new Map()}
+  private gameState : GameState = {directions: new Map(), rooms: new Map(), doors: new Map(), items: new Map(), player: {location: ''}}
   private draft : Draft<GameState> | null = null
   private onChangeListeners : ChangeListener [] = []
 
@@ -22,6 +29,25 @@ export default class Game {
     this.saveDraft()
   }
 
+  filterValidCommands(commands: ParsedCommand[]) : CommandValidateResult {
+    let validCommands : ValidCommandDetails[] = []
+    let invalidCommands : InvalidCommandDetails[] = []
+
+    for(const command of commands) {
+      const commandValidationResult = command.areNounsValid(this)
+
+      if(commandValidationResult.isValid){
+        validCommands.push(commandValidationResult)
+      } else {
+        invalidCommands.push(commandValidationResult)
+      }
+    }
+
+    invalidCommands.sort((a,b) => a.severity - b.severity)
+
+    return {validCommands, invalidCommands}
+  }
+
   onChange(callback : ChangeListener) {
     this.onChangeListeners.push(callback)
   }
@@ -51,6 +77,12 @@ export default class Game {
     return this.draft!
   }
 
+  getCurrentRoom() : Draft<Room> | null {
+    const state = this.getState()
+    return Array.from(state.rooms.values())
+    .find(room => room.name === state.player.location) || null
+  }
+
   addRoom(room : Room) {
     let state = this.getState()
     state.rooms.set(room.name, room)
@@ -65,4 +97,68 @@ export default class Game {
     let state = this.getState()
     state.items.set(item.name, item)
   }
+
+  findObjectByName(name : string, type : ObjectType) : GameObject | null {
+    let collection
+    switch(type) {
+      case ObjectType.Door:
+        collection = this.getState().doors
+        break
+        
+      case ObjectType.Item:
+        collection = this.getState().items
+        break
+      
+      case ObjectType.Room:
+        collection = this.getState().rooms
+        break
+    
+      case ObjectType.Direction:
+        collection = this.getState().directions
+        break
+    }
+
+    const objects = [...collection!.values()]
+
+    const exactMatch = objects.find((object) => name === object.name)
+    if(exactMatch)
+      return exactMatch
+    
+    const aliasMatch = objects.find(({aliases}) => aliases.includes(name))
+    if(aliasMatch)
+      return aliasMatch
+
+    return null
+  }
+
+  isVisible(object : GameObject) : boolean {
+    const state = this.getState()
+    const currentRoom = this.getCurrentRoom()
+
+    switch(object.type) {
+      case ObjectType.Direction:
+        return true
+
+      case ObjectType.Room:
+        return state.player.location === (object as Room).name
+
+      case ObjectType.Door:
+        if(!currentRoom)
+          return false;
+
+        const neighborIDs = Array.from(currentRoom.neighbors.values())
+
+        let neighbors = neighborIDs.map(name => state.doors.get(name)).filter(object => object !== undefined).map(o => o!)
+        for(const neighbor of neighbors)
+          if(neighbor.name === object.name)
+            return true;
+        return false
+
+      case ObjectType.Item:
+        return state.player.location === (object as Item).location
+
+      default:
+        return false
+    }
+  }
 }
diff --git a/src/engine/Parser.ts b/src/engine/Parser.ts
index a6cfb54..bcdb365 100644
--- a/src/engine/Parser.ts
+++ b/src/engine/Parser.ts
@@ -14,10 +14,14 @@ export default class Parser {
   }
 
   handleCommand(rawCommand : string) {
+    // Parse command for syntactical validity
+    // (according to known verb templates)
     const grammaticalParsings : ParsedCommand[] = this.parseCommandString(rawCommand)
-    console.log(grammaticalParsings)
 
-    // const validParsings : ParsedCommand[] = this.game.filterCommandsForValidity(grammaticalParsings)
+    // Ask the game state container to filter commands for object validity
+    // (nouns refer to valid objects, all objects are visible, etc)
+    const validationResult = this.game.filterValidCommands(grammaticalParsings)
+    console.log(validationResult)
   }
 
   parseCommandString(rawCommand: string): ParsedCommand[] {
diff --git a/src/engine/types/GameState.ts b/src/engine/types/GameState.ts
index 8ab7e82..4968dec 100644
--- a/src/engine/types/GameState.ts
+++ b/src/engine/types/GameState.ts
@@ -1,8 +1,11 @@
 type GameState = {
-  readonly directions: Map<ObjectID, GameObject>,
-  readonly rooms: Map<ObjectID, GameObject>,
-  readonly doors: Map<ObjectID, GameObject>,
-  readonly items: Map<ObjectID, GameObject>
+  readonly directions: Map<ObjectID, Direction>,
+  readonly rooms: Map<ObjectID, Room>,
+  readonly doors: Map<ObjectID, Door>,
+  readonly items: Map<ObjectID, Item>,
+  readonly player: {
+    readonly location: ObjectID
+  }
 }
 
 export default GameState
@@ -16,21 +19,29 @@ export enum ObjectType {
 
 type ObjectID = string
 
-type GameObject = {
+export type GameObject = {
   readonly type : ObjectType,
   readonly name : ObjectID,
   readonly aliases : string[]
 }
 
+export type Direction = GameObject & {
+  readonly type : ObjectType.Direction
+}
+
 export type Room = GameObject & {
+  readonly type : ObjectType.Room,
   readonly neighbors : Map<ObjectID, ObjectID>
 }
 
-export type Door = Room & {
+export type Door = GameObject & {
+  readonly type : ObjectType.Door,
+  readonly neighbors : Map<ObjectID, ObjectID>
   readonly locked : boolean,
   readonly key : ObjectID
 }
 
 export type Item = GameObject & {
+  readonly type : ObjectType.Item,
   readonly location: ObjectID | 'inventory'
 }
diff --git a/src/engine/types/ParsedCommand.ts b/src/engine/types/ParsedCommand.ts
index 1e54d06..d9480ce 100644
--- a/src/engine/types/ParsedCommand.ts
+++ b/src/engine/types/ParsedCommand.ts
@@ -1,6 +1,7 @@
-import { ObjectType } from "./GameState"
+import { ObjectType, GameObject } from "./GameState"
 import NounPosition from "./NounPosition"
 import Verb from "./Verb"
+import Game from "../Game"
 
 export enum TokenType {
   Expression = 'expression',
@@ -37,6 +38,14 @@ export class ParsedTokenExpression extends ParsedToken {
   }
 }
 
+export enum ParsingErrorSeverity {
+  NotVisible = 0,
+  NoSuchObject = 1
+}
+
+export type ValidCommandDetails = {isValid: true, command: ParsedCommand, subject: GameObject | null, object: GameObject | null}
+export type InvalidCommandDetails = {isValid: false, command: ParsedCommand, reason: string, severity: ParsingErrorSeverity}
+
 export default class ParsedCommand {
   readonly verb : Verb
   private tokens : ParsedToken[] = []
@@ -50,4 +59,42 @@ export default class ParsedCommand {
     newCommand.tokens = [token, ...this.tokens]
     return newCommand
   }
+
+  areNounsValid(game : Game) : ValidCommandDetails | InvalidCommandDetails {
+    const nouns = this.tokens.filter(({type}) => type === TokenType.Expression).map(token => token as ParsedTokenExpression)
+
+    let subject : GameObject | null = null
+    let object : GameObject | null = null
+    
+    for(const noun of nouns) {
+      let gameObject = game.findObjectByName(noun.name, noun.itemType)
+      if(!gameObject)
+        return {
+          isValid: false,
+          command: this,
+          reason: `You used the word ${noun.name} as if it was a ${noun.itemType}, but there is no such object`,
+          severity: ParsingErrorSeverity.NoSuchObject
+        }
+
+      if(!game.isVisible(gameObject))
+        return {
+          isValid: false,
+          command: this,
+          reason: `You cannot see ${noun.name}`,
+          severity: ParsingErrorSeverity.NotVisible
+        }
+
+      if(noun.sentencePosition === NounPosition.Subject)
+        subject = gameObject
+      else
+        object = gameObject
+    }
+
+    return {
+      isValid: true,
+      command: this,
+      subject,
+      object
+    }
+  }
 }
diff --git a/src/index.tsx b/src/index.tsx
index 8bd42d4..7e71a29 100644
--- a/src/index.tsx
+++ b/src/index.tsx
@@ -16,6 +16,16 @@ parser.understand('look')
 parser.understand('lookDirection')
   .as('look [direction]')
 
+parser.understand('lookAt')
+  .as('look [item]')
+  .as('look [door]')
+  .as('look at [item]')
+  .as('look at [door]')
+  .as('examine [item]')
+  .as('examine [door]')
+  .as('x [item]')
+  .as('x [door]')
+
 parser.understand('go')
   .as('go [direction]')
   .as('[direction]')
@@ -76,6 +86,8 @@ game.addItem({
   location: 'entry'
 })
 
+game.getState().player.location = 'entry'
+
 game.saveDraft()
 
 
diff --git a/tsconfig.json b/tsconfig.json
index f2850b7..4acaa3a 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -17,7 +17,8 @@
     "resolveJsonModule": true,
     "isolatedModules": true,
     "noEmit": true,
-    "jsx": "react"
+    "jsx": "react",
+    "downlevelIteration": true
   },
   "include": [
     "src"