Questions that the biologist must ask and answer before he can go far in generalization appear at first sight to be very numerous and varied, but, from a certain distance, we see that there are only four: What is this living creature as regards form and structure? How does it work? Whence has it arisen? How has it come to be as it is?

Introduction to Science, Classification of the Sciences (p. 110), Henry Holt & Co. 1911