1. Under the word Thought, I include all that is in us and of which we are immediately conscious. Thus all operations of the will, intellect, imagination, and senses are thoughts. But I have added 'immediately' so as to exclude those things that are their consequences. For example, voluntary motion has thought for its starting point, but in itself it is still not thought.
2. By the word Idea, I understand the specific form (fonna) of a thought, through the immediate perception of which I am conscious of that same thought. So whenever I express something in words while understanding what I am saying, this very fact makes it certain that there is in me the idea of that which is meant by those words. And so I do not apply the term 'ideas' simply to images depicted in the fantasy; indeed I do not here term these 'ideas' at all, insofar as they are depicted in the corporeal fantasy (i.e., in some part of the brain) but only insofar as they communicate their form to the mind itself when this is directed toward that part of the brain.
3. By the objective reality of an idea, I understand the being of that which is presented through the idea, insofar as it is in the idea. 21 In the same way one can speak of 'objective perfection' or 'objective art', etc. For whatever we perceive as being in the objects of ideas is objectively in the ideas themselves.
4. When things are, in themselves, such as we perceive them to be, they are said to be fonnally in the objects of ideas, and eminently when they are not just such in themselves as we perceive them to be but are more than sufficient to account fully for our perception. Note that when I say that the cause contains eminently the perfections of its effect, I mean that the cause contains the perfections of the effect with a higher degree of excellence than does the effect itself. See also Axiom 8.
5. Everything in which there is something that we perceive as immediately inhering in a subject, or through which there exists something that we perceive (i.e., some property, quality or attribute whose real idea is in us), is called substance. For of substance itself, taken precisely, we have no other idea than that it is a thing in which there exists formally or eminently that something which we perceive (i.e., that something which is objectively in one of our ideas).
6. Substance in which thought immediately inheres is called Mind (Mens). I here speak of 'Mind' rather than 'Soul' (anima) because the word 'soul' is equivocal, and is often used to mean a corporeal thing.
7. Substance that is the immediate subject of extension and of accidents that presuppose extension, such as figure, position, and local motion, is called Body. Whether what is called Mind and what is called Body is one and the same substance, or two different substances, is something to be enquired into later.
8. Substance that we understand through itself to be supremely perfect, and in which we conceive nothing at all that involves any defect or limitation of perfection, is called God.
9. When we say that something is contained in the nature or conception of some thing, that is the same as saying that it is true of that thing or can be truly affirmed of it. 22
10. Two substances are said to be distinct in reality when each one can exist without the other. We have here omitted the Postulates of Descartes because in what follows we do not draw any conclusions from them. But we earnestly ask readers to read them through and to think them over carefully. 23