Classe base tem tipo incompleto

O que exatamente esse erro significa e como posso corrigi-lo? Eu tentei encaminhar declarando a classe fazendoclass Entity no meu cabeçalho EntityPhysics mas não funcionou.

Aqui está minha Entidade.h

#ifndef __Game__Entity__
#define __Game__Entity__

#include <iostream>
#include <string>

#include "OGRE/Ogre.h"

#include "OgreInit.h"

class Entity{
public:
    Entity(std::string entityId, std::string mesh, Ogre::Vector3 position = Ogre::Vector3::ZERO, Ogre::Vector3 rotation = Ogre::Vector3::ZERO);
    virtual ~Entity() = 0;

    void setPosition(Ogre::Vector3 position);
    Ogre::Vector3 getPosition();
    void setRotation(Ogre::Vector3 rotationIncrease);
    Ogre::Vector3 getRotation();
    void setMesh(std::string meshName);
    std::string getMesh();
    virtual void tick() = 0;
    void removeEntity();

    Ogre::Entity getEntity();
    Ogre::SceneNode getSceneNode();

    std::string entityId;
protected:
    Ogre::Entity *ent;
    Ogre::SceneNode *nod;
};

#endif /* defined(__Game__Entity__) */

E meu EntityPhysics.h

#ifndef __Game__EntityPhysics__
#define __Game__EntityPhysics__

#include <iostream>
#include <string>
#include "OGRE/Ogre.h"
#include "OgreBulletCollisionsBoxShape.h"
#include "OgreBulletDynamicsRigidBody.h"

#include "Entity.h"
#include "OgreInit.h"

class EntityPhysics: public Entity //error occurs here: "Base class has incomplete type"
{
public:
    EntityPhysics(std::string pentityId, std::string mesh, Ogre::Vector3 position, Ogre::Vector3 rotation, /*Physics Specific "stuff"*/std::string shapeForm = "BoxShape", float friction = 1.0, float restitution = 0.0, float mass = 1.0);
    virtual ~EntityPhysics() = 0;
    virtual void tick() = 0;
private:
    float friction, restitution, mass;

    OgreBulletCollisions::CollisionShape *collisionShape;
    OgreBulletDynamics::RigidBody *rigidBody;
};

#endif /* defined(__Game__EntityPhysics__) */

Eu acho que pode ter a ver comigo inclusiveEntity.h na classe filho, mas se eu fizer isso, recebo o mesmo erro.