Thought, memory, will and reason, together with the ego as the fifth and principal factor, constitute the heart. It is these five things that may be called the heart, but in definitely naming the different parts of this heart we call the surface of it mind, and the depth of it heart.

As the surface of the heart is known by the imagination and thought, so the depth of the mind, which is the heart, is known by feeling.

The difference between thought and imagination is that imagination is an automatic working of the mind. If the mind is fine there is a fine imagination; if the mind is gross there is a gross imagination; if there is a beautiful mentality, the imagination is beautiful.
Thought is also imagination, but imagination held, controlled, and directed by will. Therefore when we say: "He is a thoughtful person", it means that this person does not think, speak, or act on impulse, but behind everything he does there is will-power which controls and directs the action of his mind.

The ego stands separate from the four faculties of thought, memory, will and reason. It is just like four fingers and a thumb. Why is the thumb not called a finger? Because the thumb is the whole hand. These four are faculties, but the ego is a reality. It holds and accommodates within itself the four faculties, and in order to distinguish it as different from them we call it ego.