EVIDENCE FOR CONDITIONAL EXISTENCE CLAIMS

Now, many of the claims in theories of the sort Mill is con­cerned with have the form:

(3) As have P (light waves in the ether travel in a direction perpendicular to the wave front),

where this is supposed to be understood as being elliptical for, or presupposing, (2).

However, what is often presented as evidence for a claim of form (3) is really evidence for a weaker conditional claim of form (1). Thus, Mill wants to say, even if the estab­lished fact that light moves in straight lines is evidence for the conditional claim (1), it is not evidence for the uncondi­tional claim about light waves and the ether in (2). And the latter is one of the fundamental assumptions of the wave theory.

So, for Mill, even if there is evidence for various assumptions of the wave theory, provided that those assumptions are construed conditionally on the ether and light waves existing, they are not evidence for unconditional versions of those assumptions. Yes, Mill will agree, the con­ditional assumptions are part of the theory, and yes, there can be evidence for them. But so are their unconditional versions. And unless there is evidence for them, the theory as a whole is a speculation. That is why Mill focuses on the lack of evidence for the existence of the ether. Without this, at best you can have evidence for conditional claims in the wave theory, but not the unconditional ones. And it is the latter that wave theorists most want to assert.

For Mill, the most you can say is that there is evidence that the theory is a possibility, that it might be true, that it is worth exploring—which is exactly what Mill does say about the wave theory of light.

It is perfectly consistent with the spirit of the [deductive] method, to assume in this provisional manner not only an hypothesis respecting the law of what we already know to be the cause but an hypothesis respecting the cause itself. It is al­lowable, useful, and often even necessary, to begin by asking ourselves what cause may have produced the effect, in order that we may know in what direction to look for evidence to determine whether it actually did?²

Newton, who held a methodological view (“analysis and synthesis”) similar to Mill's, is also a particularist who demands that for a theory to be proved, each of its claims needs to be established by causal-inductive reasoning or by deduction from other claims so established. (For Newton, you can't establish the law of gravity simply by showing that if gravitational forces exist that obey the law of gravity, then these forces produce known planetary motion. You have to show by causal-inductive reasoning that gravitational forces obeying his law do exist, and do in fact produce the motions in question.) Newton, at least in his official meth­odology, is even stricter than Mill. For Newton, if a theory is

22. Reprinted in Achinstein, Science Rules, 222. not so established, then it is what he calls a “hypothesis,” and “hypotheses have no place in experimental philosophy.”^[128]

13.