m. 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 said, that a finger is short and longor short and not‑short, and long and not‑longdoes not contravene the formal logical principle of non‑contradiction.
Of very great interest are categories of propositions at what may be, termed different levels of abstraction, together with the connections between them. For example, to describe and distinguish things in terms of their sensible qualities (such as colour) is to operate at a certain level of abstraction. But information about what produces these sensible differences (for instance, what makes things look one colour rather than another) belongs to another level of abstraction and is formulated in propositions of different category. And the second category of information does not replace the first, but connects with it. Together these different categories of information constitute information about (in the case of colour) how bodies reflect light and how that affects our senses. This kind of connection of propositions at different levels of abstraction and of different category is of great importance in the management of information. For on it depends the proper assembly of relatively abstract items of information into something more concrete‑filling in and generalising our conceptions of the matters that concern us.
All information is abstract (how could it not be?). And what I have just called the management of information (which is the practical business of thinking, as distinct from what Engels called "idealist fancy") consists of assembling abstractions. Information is not only negative and positive (for example, "This rose is not white" and "This rose is red"), and more or less generalised ("All roses have thorns" and "All red roses have thorns"), and more or less abstract ("This rose is red" and "This rose is red and sweet‑smelling")all of which are differences of form and are treated in formal logic. It also exemplifies a host of what may be called different modes of abstraction, which are not distinguished in formal logic because they are not formal difference