_b_) The propositions of physical science are not found among the data of apprehension. Mathematical propositions may be disposed of by making them purely a priori; propositions about sense objects by making them purely a posteriori.[95] But physical propositions, such as make up physics, chemistry, biology, to say nothing of propositions of history, anthropology, and society, are neither one nor the other. I cannot state the case better than Mr. Russell has stated it, although, I am bound to add, the stating did not arouse in Mr. Russell any suspicion of the premises with which he was operating. "Men of science, for the most part, are willing to condemn immediate data as 'merely subjective,' while yet maintaining the truth of the physics inferred from those data. But such an attitude, though it may be _capable_ of justification, obviously stands in need of it; and the only justification possible must be one which exhibits matter as a logical construction from sense data.... It is therefore necessary to find some way of bridging the gulf between the world of physics and the world of sense."[96] I do not see how anyone familiar with the two-world schemes which have played such a part in the history of humanity can read this statement without depression. And if it occurred to one that the sole generating condition of _these_ two worlds is the assumption that sense objects are modes of apprehension or knowledge (are so intrinsically and not in the use made of them), he might think it a small price to pay to inquire into the standing of this assumption. For it was precisely the fact that sense perception and physical science appeared historically (in the seventeenth century) as rival modes of knowing the same world which led to the conception of sense objects as "subjective"--since they were so different from the objects of science. Unless sense and science had both first been thought of as modes of knowing and then as modes of knowing the same things, there would not have been the slightest reason for regarding immediate data, as "merely subjective." They would have been natural phenomena, like any other. That they are phenomena which involve the interaction of an organism with other things is just an important discovery about them, as is also a discovery about starch in plants.