An Essay Concerning Human Understanding/Book IV/Chapter XXI
1. Science may be divided into three sorts. All that can fall within the compass of human understanding, being either, First, the nature of things, as they are in themselves, their relations, and their manner of operation : or, Secondly, that which man himself ought to do, as a rational and voluntary agent, for the attainment of any end, especially happiness : or, Thirdly, the ways and means whereby the knowledge of both the one and the other of these is attained and communicated ; I think science may be divided properly into these three sorts : –
2. Physica. First, The knowledge of things, as they are in their own proper beings, their constitution, properties, and operations ; whereby I mean not only matter and body, but spirits also, which have their proper natures, constitutions, and operations, as well as bodies. This, in a little more enlarged sense of the word, I call Phusike, or natural philosophy. The end of this is bare speculative truth : and whatsoever can afford the mind of man any such, falls under this branch, whether it be God himself, angels, spirits, bodies ; or any of their affections, as number, and figure, &c.
3. Practica. Secondly, Praktike, The skill of right applying our own powers and actions, for the attainment of things good and useful. The most considerable under this head is ethics, which is the seeking out those rules and measures of human actions, which lead to happiness, and the means to practise them. The end of this is not bare speculation and the knowledge of truth ; but right, and a conduct suitable to it.
4. Semeiotike. Thirdly, the third branch may be called Semeiotike, or the doctrine of signs ; the most usual whereof being words, it is aptly enough termed also Logike, logic : the business whereof is to consider the nature of signs, the mind makes use of for the understanding of things, or conveying its knowledge to others. For, since the things the mind contemplates are none of them, besides itself, present to the understanding, it is necessary that something else, as a sign or representation of the thing it considers, should be present to it : and these are ideas. And because the scene of ideas that makes one man’s thoughts cannot be laid open to the immediate view of another, nor laid up anywhere but in the memory, a no very sure repository : therefore to communicate our thoughts to one another, as well as record them for our own use, signs of our ideas are also necessary : those which men have found most convenient, and therefore generally make use of, are articulate sounds. The consideration, then, of ideas and words as the great instruments of knowledge, makes no despicable part of their contemplation who would take a view of human knowledge in the whole extent of it. And perhaps if they were distinctly weighed, and duly considered, they would afford us another sort of logic and critic, than what we have been hitherto acquainted with.
5. This is the first and most general division of the objects of our understanding. This seems to me the first and most general, as well as natural division of the objects of our understanding. For a man can employ his thoughts about nothing, but either, the contemplation of things themselves, for the discovery of truth ; or about the things in his own power, which are his own actions, for the attainment of his own ends ; or the signs the mind makes use of both in the one and the other, and the right ordering of them, for its clearer information. All which three, viz, things, as they are in themselves knowable ; actions as they depend on us, in order to happiness ; and the right use of signs in order to knowledge, being toto coelo different, they seemed to me to be the three great provinces of the intellectual world, wholly separate and distinct one from another.