Our task is the simpler one of the philosophy of the sciences.

Now a science has already a certain unity which is the very reason why that body of knowledge has been instinctively recognised as forming a science.

The philosophy of a science is the endeavour to express explicitly those unifying characteristics which pervade that complex of thoughts and make it to be a science.

The philosophy of the sciences--conceived as one subject--is the endeavour to exhibit all sciences as one science, or--in case of defeat--the disproof of such a possibility.
Again I will make a further simplification, and confine attention to the natural sciences, that is, to the sciences whose subject-matter is nature.

By postulating a common subject-matter for this group of sciences a unifying philosophy of natural science has been thereby presupposed.
What do we mean by nature?
We have to discuss the philosophy of natural science.