2. The non-resembling species in the genus x show no initial tendency towards y.
∴ The non-resemblances observed are such as could not produce protective resemblances. This is a formally bad argument from two negative premisses justified by its positive meaning, which implies that just where the alleged effect ceases, the alleged cause ceases too.
If you look at the case in the Natural History Museum [1] you see the normal Pierinae down one side, not approaching Euploinae. They are the positive examples, negatively confirming the explanation of those which do approach Euploinae. These latter all start from some form which varied slightly, by accident we presume, towards Euploinae, and then this partially resembling series splits into three sets, each leading up to a different and complete protective resemblance.
[1] These cases in the entrance-hall of the Natural History Museum at South Kensington afford excellent practical illustrations of Inductive Method. I strongly urge the London student to try his hand at formulating them.
I said mere number was no help in scientific Induction. But do not these three sets of resemblances make a stronger proof than any one would? Yes, because we need a presumption against accident. You would not want this if you could unveil what really happens in one case, but as infinite conditions are operative in such matters, and it is impossible to experiment accurately, [1] this cannot be done; {158} and it might be said that one such resemblance was an accident, i.e. that it was owing to causes independent of the protection. But as the cases become more numerous it becomes more improbable that different circumstances produce the same effect, which would then be a mere coincidence, in so many different cases. If, however, we knew by positive and negative analysis what circumstance did produce the effect, this confirmation would be useless.
[1] Ultimately, no experiments are absolutely accurate. There is always an unexhausted background in which unsuspected causes of error may be latent.
Negative Instance
(f) In order to show exactly what circumstance produces a given effect, a system must be brought to bear on the phenomenon through negation. The only test of truth is that it is that which enables you to organise your thought and perception.
The first means of doing this is Observation, then Experiment, then
Classification and Hypothesis, which takes us into Deduction.
Observation is inaccurate, until you begin to distinguish what is connected from what is not connected. When you do this, you are very near experiment, the use of which is to introduce perfectly definite and measurable changes into what you are observing. [1] There is no absolute distinction between observation and experiment. Looking at a tissue through a microscope is observation; putting on a polariscope, though it changes the image altogether, is observation; if you warm the stage, or put an acid on the object, that, I suppose, is experiment, because you interfere with the object {159} itself. What should we say, for example, as to spectroscopic analysis of the Sun’s corona?