The literature of our day shows experimental scientists to be divided between two schools of thought, now generally called Mechanist and Vitalist. The literature of any day these last 2000 years would tell the same tale, but for occasional changes of name. Where an issue dividing scientists is seen to be an experimental issue, it presents no challenge to the philosopher. His interest is limited to the question, How shall we find out? and where all are agreed as to the way of settling a difference of opinion, he can wait with patience to learn the result. But the very history of this world-old and world-wide conflict between schools of experimental science shows that it never has been, is not now, could never become one whose issue turned on the outcome of this or that experiment; nor in all the 2000 years the conflict has lasted, has science been able to come upon any other type of evidence by which the issue might be settled. As one of a class of similar issues, ancient, pervasive, persistent, the manner of whose decision lies still beyond the grasp of experimental science, this Mechanist-Vitalist controversy gives the philosopher much to think about.