If we are talking or thinking about anything, we can in some way or another attribute being to it. In attributing being to something, we can attribute it as extramental or in the mind. Set aside intramental being. We always attribute something to a subject. This attribution in the case of what is beyond the mind can be an attribution of either what the subject is, or what is in the subject, or what is beyond the subject but taken up by it. These three ways capture how we think of anything outside the mind.
(1) When what we attribute to a being outside the mind is something essential to it, we are attributing it in the mode of substance. Thus in 'Socrates is human', we are saying what it is that Socrates himself is.
If we aren't attributing it to the subject as something essential, we obviously are attributing it as something other than what is essential. This may be done independently of anything beyond the subject (as in the subject) or as dependently on what is beyond the subject (as taken up by the subject). If we are talking about the first (what is in the subject), it may be in the subject absolutely or relatively. If we are attributing it as absolutely in the subject, we may be attributing it as materially in the subject, thus making it divisible into parts, or as formally unified.
(2) When we attribute something as being in the subject materially, we are attributing it in the mode of quantity.
(3) When we attribute something as being in the subject formally, we are attributing it in the mode of quality.
(4) When we attribute something as being in the subject relatively, we are attributing it in the mode of relatedness or to-another. For instance, in 'Socrates is a father', we are giving a way in which Socrates himself has a 'to another' aspect, namely, fatherhood, which is relative to children.
These are the four primary categories, and they serve in a sense as the framework for all of our talking and thinking about existing things themselves. But we can also attribute things to a subject that depend on things other than the subject itself. In such cases, the thing external to the subject serves as a basis for thinking or talking about the subject. In order to do this, the external thing has to be linked to the subject in some way that makes it possible for us to think about the one in terms of the other. There are three major situations in which this happens: when the external thing is a cause or effect, when it is a measure, and when it is an adjunct to the subject.
(5) When we attribute something to a subject in a way that depends on an external thing as cause, we are attributing it in the mode of passion, because things undergo the actions of their causes.
(6) When we attribute something to a subject in a way that depends on an external thing as effect, we are attributing it in the mode of action, because causes act on their effects.
Because of the way causation works, predications of these two kinds, while dependent on something beyond the subject, do tell us about what it is in the subject in some sense. Attributions involving measure are more removed and indirect. We can measure something either with regard to a change or to a containing boundary.
(7) When we attribute something to a subject so as to measure it by a change, we are attributing it in the mode of when.
Measuring with respect to a containing boundary can occur in two ways, either simply or in such a way as to indicate the ordering of parts to each other.
(8) When we attribute something to a subject so as to measure it in itself with respect to a boundary, we are attributing it in the mode of where. So 'Socrates is in the agora' indicates an outer boundary (the agora) with respect to which Socrates himself can be measured.
(9) When we attribute something to a subject so as to measure it in a way that relates its parts to each other, we are attributing it in the mode of posture. So 'Socrates is sitting' indicates how Socrates's parts are related to each other with respect to the boundary around Socrates.
The third way we can attribute something to a subject based on something beyond the subject is through an adjunct. The most common and obvious way in which this happens is with clothing and clothing-like things; for instance, if we say 'Socrates is dressed', calling him dressed is with reference to something joined to him.
(10) When we attribute something to a subject based on something beyond the subject that is adjoined to it, we are attributing it in the mode of habit or vestment.
The last six of these modes, sometimes known as the sex principia, are particularly useful for thinking and talking about changing things, where figuring out what is going on often requires using other things as a framework.
To put in outline form:
The predicate of the subject characterizes
-- (I) what the subject itself is: substance
-- (II) what is in the subject itself
-- -- -- (A) materially: quantity
-- -- -- (B) formally: quality
-- -- -- (C) relatively: relation
-- (III) What is taken up by the subject from outside itself
-- -- -- (A) causally
-- -- -- -- (1) from a cause: passion
-- -- -- -- (2) from an effect: action
-- -- -- (B) as a measure
-- -- -- -- (1) with respect to a change: when
-- -- -- -- (2) with respect to a boundary
-- -- -- -- -- (a) measuring the subject itself: where
-- -- -- -- -- (b) ordering the subject's parts to each other: posture
-- -- -- (C) as an adjunct: vestment
Obviously there are many questions that can be associated with each; for instance, you can ask whether vestment literally applies only to human beings or also to things like hermit crabs or inanimate objects that are covered. (Aquinas himself thinks that cases of the latter are metaphorical applications of the category of habitus or vestment, but you could perhaps have a somewhat broader understanding of the category in which they wouldn't be.) But the categories so interpreted lay out ways in which we characterize things in the external world.
