or certifiable by procedures of formal logic. Suitably generalised (for it applies not only to roses), it may be called, after Ryle, a "category proposition".
Propositions stating colour may be termed a category of propositions, distinct from, for example, those stating shape or weight. And obviously, there are principles or rules for abstraction ("category propositions") dealing not only with abstracting one proposition of a given category from another of that category (such as deriving "This is not white" from "This is red"), but also for abstracting propositions of other categories (such as deriving "This has a shape" from "This has a colour"). To go against category rules, or "category propositions", always results in absurdity or non‑informativenesssuch as saying that "So and so's red roses are white", or "So and so's roses have no shape or size".
Such examples or the conclusions of an investigation of categories may well be regarded as trivial and obvious, so that such an investigation may be thought trivial and unnecessary. However, two points are already worth noting.
The first is that a computer has to deal with definite categories of information, and has to be instructed and programmed according to the rules for those categories. The people who build and instruct computers must know those rules before they can make machines that obey them. Computers themselves could not conclude to such rules by any process of their own mechanisms.This is an illustration of the fact that human thought, on the one hand, and what computers do and can conceivably be made to do, on the other, are by no means the same. A machine can quite well be made to observe and record observations and generalise and abstract from what it observes. But just as it does not fall in love with other machines and produce little ones, or co‑operate with other machines in socially producing and distributing their means of sustenance, so it lacks the capacity of developing for itself the categories of thought and the rules for operating with them. It is not made of the right stuff for doing such things. In an age of automation and computation it is important to work out with great care principles concerning categories as well as a calculus of formal logic.
Secondly, even some of the most apparently trivial category rules are not so obvious as they look. Take colour, for example. Information about how things are coloured includes not only such items as "This is red" but also "This looks red". Thus it can be stated: "This is red, but it looks yellow in this light". Again, how tell the colour of, say, the sunset on Mars, when there is no one there looking at it? Indeed, what does anything look like when no one is looking at it? Such considerations show that it would be mistaken to regard information about colour as information about certain simple qualities which either do or do not belong to things irrespective of their relationships with other things. Propositions specifying colour are evidently relational propositions, though this is disguised by the verbal forms in which they are often expressed. Hence there is plenty of room for painstaking investigation of the categories of propositions even in the case of such apparently trivial examples as that of propositions stating colour, and it yields conclusions which are far from obvious at first sight.
This kind of discovery of the not so obvious in the obvious was what originally led to the use of the expression "dialectics". Thus Plato illustrated dialectics by holding up a finger and asking if it was short or long. It was short in relation to longer things, and long in relation to shorter things. From this he reached the important and by no means obvious conclusion (essentially a conclusion about category, though he did not put it in these terms) that propositions specifying length are a category of relational propositions. From this, incidentally, it already becomes evident that there is no sort of incompatibility, as has sometimes been suggested, between dialectics and formal logic. To say, as Plato sa