Some modern thinkers have supposed that “cause” is an outworn notion, or at least that it is one of which modern science has no need. This is due mainly to the discovery that, while the scientist can give us general laws as to what in fact happens, he cannot help us to discern the reason for the laws or the inward nature of the forces on which they depend. He can tell us the “that” but not the “why”; he cannot show us in a single case that the effect follows necessarily a priori from the nature of the cause, that any other effect than the one which actually takes place would be logically impossible. He has studied the law of gravitation, but this law does not enable him to see why material bodies should attract each other in this fashion; it is only a generalized statement of the fact that they do. He knows that certain substances, if absorbed by eating, will nourish and others destroy our tissues; but he cannot say why they should do so. He can no doubt analyse them further and discover that, for example, meat is nourishing because it contains a large proportion of nitrogenous matter, but he could not tell a priori whether this nitrogenous matter would be likely to nourish or to poison us. Only where mathematics can be applied do we see necessity in such a way that any alternative becomes inconceivable to us; but mathematics alone can never establish from a quantity present here and now what quantity there will be at a later time or in another part of space. Mathematics can show, e.g., that, if there is 2 + 2 here and now, there must be 4 here and now, not that, if there is 2 + 2 here and now, there will be 4 in an hour's time or a mile away; and therefore it cannot be made the sole basis of any causal law whatever.
Accretion-Disk Corona Advected by External Radiation Drag