Thus the molecules and electrons of scientific theory are, so far as science has correctly formulated its laws, each of them factors to be found in nature.

The electrons are only hypothetical in so far as we are not quite certain that the electron theory is true.

But their hypothetical character does not arise from the essential nature of the theory in itself after its truth has been granted.
Thus at the end of this somewhat complex discussion, we return to the position which was affirmed at its beginning.

The primary task of a philosophy of natural science is to elucidate the concept of nature, considered as one complex fact for knowledge, to exhibit the fundamental entities and the fundamental relations between entities in terms of which all laws of nature have to be stated, and to secure that the entities and relations thus exhibited are adequate for the expression of all the relations between entities which occur in nature.
The third requisite, namely that of adequacy, is the one over which all the difficulty occurs.

The ultimate data of science are commonly assumed to be time, space, material, qualities of material, and relations between material objects.