Descartes on Substance, Principal Attributes and Modes
It is important to note that there are different and conflicting conceptions of substance in the Meditations. Two will be considered.
(a) Independence Conception
Substance as a causally independent, and subject independent, thing.
The most common conception of substance relies upon the ideas of causal independence and subject independence. Causal independence is the ability to exist, or to remain in existence, independently of another thing's causal power. Subject independence is the ability for a thing to exist without depending on another thing to provide the subject "in which" it exists, i.e to not be a property (or accident or mode) of a substance, but to be a substance in its own right.
The independence conception of substance may be divided in turn into "uncreated" (or "primary", or "infinite") substance, and "created" (or "secondary", or "finite") substance, as follows.
(1) Uncreated Independent Substance = thing that is not dependent upon the causal power of any other thing in order to exist or to remain in existence, and is not a property of any other thing.
(The only thing that satisfies this definition of "uncreated substance" is God.)
(2) Created Independent Substance = thing that is dependent upon the causal power of God to exist and to remain in existence, but is not dependent upon the power of any other thing to remain in existence, and is not a property of any other thing.
(The things that satisfy this definition of "created substance" are angels, souls (minds) and individual bodies.)
Descartes sometimes talks as though, properly speaking, the only substance is God. Buy he never fails to distinguish between God and what God has created.
(b) Pure Conception
However, in other parts of the Meditations, Descartes appears to have another conception of substance that relies upon the idea of mereological independence, that is, not being divisible into parts, and hence, being pure. Hence substances may be distinguished as "pure" and "impure".
(1) Pure Substance = an uncreated or created independent substance that cannot be divided into parts.
(The things that satify this definition of substance are (i) God, (ii) angels, (iii) souls (minds), and (iv) ALL EXTENDED SUBSTANCE TAKEN AS A WHOLE (i.e. all matter taken as one gigantic body, which is nevertheless a finite amount).
(2) Impure Substance = a created independent substance that can be divided into parts.
(The things that satisfy this definition of substance are individual bodies, including each human body.)
Some points to note about the pure/impure distinction are as follows.
First of all, pure substances are not subject to decay (i.e. breaking up into parts). Created pure substances -- which are by definition created by God in the first place -- may be annihilated by God, but that is about it. They are immortal, unless God decides to annihilate them. (God, of course, does not, indeed cannot, annihilate himself, since He is a necessarily existing being.)
Secondly, pure substances remain the same even if they lose all of their properties and gain new ones. So, for example, even if a soul loses all of her thoughts, including all of her memories, she is still the same soul. (Descartes does not have anything further to say here about personal identity, so long as there is substance identity).
Thirdly, individual extended substances merely disperse into different arrangements (e.g. apple rots into other form of extended substance with different properties). These changes from one form to another, however, all occur using the same total amount of extended substance. None is ever lost or gained. There is a strict conservation of extended substance.
Fourthly, although Descartes does not discuss the subject, it does appear to be possible for God to annihilate some individual created extended subsance (i.e. annihilate some piece of the total), or even, perhaps, to annihilate all of them (i.e. annihilate all of it). If He did, the world would simply close up around it. It also seems possible for God to create some new individual extended substance (i.e. add some more to the total) than He created to begin with. If He did, the rest of the world would be 'split open' to accommodate this new individual extended substance (i.e. new part). In this unlikely eventuality (why would God change His mind?) it would follow that there would be a NEW extended substance, taken as a whole, since a new part would be added to the one extended substance.
Principal Attributes and Modes
Although Descartes distinguishes between substance and non-substance absolutely, he does distinguish between the different sorts of things that are not substances and that are dependent upon substance as subjects "in which" to exist. These are principal attributes and modes. (The expression "principal attribute" is used because Descartes often refers to modes as attributes. Modes may also be called accidents and in general, properties.)
Each substance has one, and only one, principal attribute. An attribute makes a substance the type of substance that it is. It is defined in his Principles of Philosophy as that which "constitutes its nature and essence, and to which all its other properties are referred".
In the case of God, angels and souls (minds), the principal attribute is thought.
In the case of bodies, or even all matter taken as one body, the principal attribute is extension.
This is Descartes's dualism: there are two attributes, corresponding to two types of substance, thinking substance and extended substance.
There is, then, a one : one relation between substance and and attribute for Descartes. This is because each substance has one nature or essence, that is, is one type of substance, and one way in which its nature or essence is constitited, that is, one atttribute.
In addition, every substance has modes, or properties. A mode is a "modification" of a substance; always, however, a modification of its attribute.
The modes of God, angels and souls (minds) are thoughts. There is a broad and narrow interpretation of this. Broadly, for example, God's "omniscience" is a mode of thought. Indeed, all His perfections are modes of thought, since he is nothing except a thinking substance. Narrowly, for example, my particular feeling of pain here and now is a mode of thought.
The modes of individual bodies, and of all extended substance taken as one body, are three-dimensional shapes, sizes, and locations. There are also the modes of colors, smells, etc.; however, Descartes does not consider these modes to be truly in the body.
While a soul may change all of its modes for other modes, i.e. change all of its thoughts, it cannot change its attribute, i.e. thinking. It cannot be without its attribute, and hence, without some mode or other. The same is true for a body.
Descartes holds that we can distinguish between a substance, its attribute, and its mode. However, he holds that the distinction between a substance and is attribute is "merely a conceptual distinction" or a "distinction of reason" since we cannot intelligibly grasp a substance without its attribute (e.g. we cannot intelligibly grasp a soul without thought). The distinction between a substance and its mode (e.g. a soul and its feeling fear) is a "modal distinction".