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we should know which hypothesis in each case it is best to keep in mind ; 
—to ascertain which, in respect to the subject in hand, was my only object 
in the paper laid before you. Then as to that unlucky, I must say very 
unlucky, analogy of gravitation. I say very unlucky, not because I disbelieve 
gravitation any more than I did, but because that part of my paper has been 
so absolutely misunderstood. It has been dealt with as if I made a distinct 
comparison between gravitation and Darwinism, and used the one in some 
way to support the other. (Hear, hear.) What have I done ? I have done 
this I had occasion to employ four technical terms, each capable of very 
large and diverse use, and I wanted every one to understand exactly what I 
meant by those terms. I thought, How shall I do this ? Lawyers, I believe, 
are finding out that it is wiser, instead of attempting to frame precise defi- 
nitions, rather to give illustrations ; and so I thought I would do the same, 
and illustrate rather than define the meaning I assigned to these technical 
terms. I had in my head two illustrations equally adapted to my purpose 
the one I actually used, of gravitation, and the one which Mr. Mitchell (who 
though he hates hypotheses, yet sometimes uses them with good effect) gave 
us the other evening about meteors. I distinctly thought of putting that in as 
my illustration. Clearly then for Mr. Reddie to go through the various points 
in which he thought there was a resemblance between Darwinism and gravi- 
tation was throwing time away. Let me quote the exact words I used about 
this illustration of gravitation. I said : “ The precise meaning to be attached 
to these terms, and the value to be set upon the tests they denote, may be 
best seen by a simple example.” I pass by, therefore, all that Mr. Reddie 
has said about these terms as applied to gravitation. They have nothing 
to do with what I have said. They are interesting to those who want to go 
into the subject of gravitation, but they have nothing whatever to do with 
Darwinism. Then as to Neptune, what was the point I was arguing ? It 
was this It is perfectly conceivable that there may be a flaw in our 
evidence concerning any explanation of a phenomenon, which arises, not 
from a real absence of evidence, but from our being unable to get at it. 
Every theologian puts it down as one of the canons of criticism that when- 
ever a possible explanation, of any contradiction between passages of Scrip- 
ture can be given, although you cannot prove one iota of that explanation to 
be true, the force of the objection is thereby destroyed, since it is shown that 
there may be, after all, no contradiction at all. I say that principle is iaid 
down by theological advocates as one of the primary canons of criticism. 
I believe the canon is a true one in science also, though I do not say that 
when such flaws in the evidence exist, the -hypothesis stands on the same 
level as before. Let me read my words again on that point. I said : “ It 
is plain, then, that no objection to a hypothesis should be regarded as of 
final weight for which a possible explanation can be given not inconsistent 
with observed facts. Weaken the credibility of the hypothesis such objections 
can and do ; destroy it altogether they cannot.” And I submit, however 
much the thing may be laughed at, that so long as the fact remains that a 
planet could exist, and could roll round the sun, without being possibly visible 
