At one point Aristotle called mind a 'sort of soul', at another point a 'part of soul' and at another, a form of soul. His best explanation said that the soul is like a mathematical series, only one in which each succeeding term is dependant on those preceding. He said, for example, that plants and animals have an appetitive soul because they both must absorb food. Animals also have a sensitive soul but having an appetitive soul is necessary to having a sensitive soul. Mind, on the other hand only exists in those animals which have both the appetitive and the sensitive soul. In other words, each term requires the existence of those below it. But whether these definitions deal with separable units or not he has not determined.

Thinking, Aristotle said, is like perception except thinking can be in error. But let us look at Aristotle's words concerning mind and thinking.

Turning now to the part of the soul which the soul knows and thinks (whether this is separable from the others in definition only or spatially as well) we have to inquire (1) what differentiates this part, and (2) how thinking can take place.

If thinking is like perceiving it must be a process in which the soul is acted upon by what is capable of being thought, or a process different from but analogous to that. The thinking part of the soul must therefore be, while impassable, capable of receiving the form of an object; that is, must be potentially identical in character with its object without being the object. Mind must be related to what is thinkable, as sense is to what is sensible.

This relationship between what is thought of and the thought of it has always posed a problem for philosophers. Aristotle avoided the specifics of the relationship while making clear that such a relationship must exist and that relationship must be specific to what is being thought. Remember, that the soul and the body of an existing living entity cannot be separated except in thought any more than the form of a non-living object can be separated from the object itself. Therefore the mind, in contemplating existing objects extracts from our experience of these objects the essence or form of that which the object is. In the case of living objects this is the soul. That means that the mind must have the ability to become itself the forms of that which it thinks of. This makes the souls of living things thinkable entities and not physical entities.

Therefore, since everything is a possible object of thought, mind in order, as Anaxagorus says, to dominate, that is, to know, must be pure from all admixture; for a copresence of what is alien to its nature is a hindrance and a block; it follows that it too, like the sensitive part, can have no nature of its own other than that of having a certain capacity. Thus, that in the soul which is called mind (by mind I mean that whereby the soul thinks and judges) is, before it thinks, not actually any real thing. For this reason it cannot reasonably be regarded as blended with the body; if so it would acquire some quality, e.g. warmth or cold, or even have an organ like the sensitive faculty.

While the faculty of sensation is dependent on the body, mind is not because following the strong stimulation of a sense we find ourselves less able to utilize that sense. On the other hand, thought about an object that is highly intelligible results in an increase in our ability to think. The accumulation of knowledge such as in science is accomplished by the mind, through the process of extraction, becoming the form of each of the possible objects. Once an accumulation has been developed mind is able then to think about itself. Then Aristotle posed this question. If mind is thinkable and what is thinkable is in kind one and the same then mind will either belong to everything, or will contain something in common with all other realities which make them thinkable. His answer was that mind is potentially whatever is thinkable though it is nothing until it has thought. What it thinks must be in it in the same way that characters are in a writing tablet prior to anything being written on it.

Michael Wedin brought out some interesting points regarding the relationship between mind and imagination in Aristotle. Imagination according to Aristotle is that mechanism by which we produce images of the things we perceive. Wedin chose this paragraph from De Anima as what he called Aristotle's 'canonical theory of imagination.'

But since it is possible that when one thing is moved a different thing is moved by it, and since imagination is thought to be a sort of movement, and is thought not to occur apart from perception but in perceiving things, and in relation to that of which there is perception, and since it is possible for movement to occur as a result of actual perception, and since this movement is necessarily like the perception, this movement cannot exist apart from the perception or in things that are not perceiving things. And the possessor of the movement may do and be affected by many things in respect of it, and it may be true or false.

Without getting into Wedin's arguments for his position, what he maintained was that Aristotle's concept of imagination is an image producing mechanism that is included in the body and tied to the senses. However, he went on to say that Aristotle also made that same imagination the image producing agent of the mind. Above we saw that Aristotle made mind free of connection to the body. Thus, the mind is eternal or immortal. But its connection to imagination makes it such that it loses its potency once the body is dead. He said later that the intellect, or that by which we recognize first principles and valid arguments, is eternal, which left one more door open for later Christians to put Aristotelian ideas into a Christian context.