It is that the first task of a philosophy of science should be some general classification of the entities disclosed to us in sense-perception.
Among the examples of entities in addition to 'events' which we have used for the purpose of illustration are the buildings of Bedford College, Homer, and sky-blue.

Evidently these are very different sorts of things; and it is likely that statements which are made about one kind of entity will not be true about other kinds.

If human thought proceeded with the orderly method which abstract logic would suggest to it, we might go further and say that a classification of natural entities should be the first step in science itself.

Perhaps you will be inclined to reply that this classification has already been effected, and that science is concerned with the adventures of material entities in space and time.
The history of the doctrine of matter has yet to be written.

It is the history of the influence of Greek philosophy on science.