In the first place, as Dr. Venn shows, natural phenomena want the distinctness and capability of isolation that belong to symbols.  Secondly, the observing whether instances conform to a Canon, must always be subject at last to the limits of our faculties.  How can we ascertain exact equality, immediate sequence?  The Canon of Difference, in its experimental application, is usually considered the most cogent sort of proof:  yet when can the two sequent instances, before and after the introduction of a certain agent, be said to differ in nothing else?  Are not earth and stars always changing position; is not every molecule in the room and apparatus always oscillating?  It is true that our senses are now aided by elaborate instruments; but the construction of these depends on scientific theories, which again depend on experiments.

It is right to touch upon this well-known sceptical topic; but to insist much upon it is not a sign of good sense.  The works of Herschel, Whewell, and Jevons should be consulted for the various methods of correcting observations, by repeating them, averaging them, verifying one experimental process by another, always refining the methods of exact measurement, multiplying the opportunities of error (that if any exist it may at last show itself), and by other devices of what may be called Material Logic or Methodology.  But only direct experience and personal manipulation of scientific processes, can give a just sense of their effectiveness; and to stand by, suggesting academic doubts, is easier and more amusing.

Sec. 2.  Still, it is not so much in laws based upon direct observation or experiment, that the material validity of scientific reasoning appears, as in the cumulative evidence that arises from the co-ordination of laws within each science, and the growing harmony and coherence of all sciences.  This requires a more elaborate combination of deduction with observation and experiment.  During the last three hundred years many departments of science have been reduced under principles of the greatest generality, such as the Conservation of Energy, the Law of Gravitation, the Undulatory theory of Light, the Law of combining Equivalents, and the Theory of Natural Selection; connecting and explaining the less general laws, which, again, are said to connect and explain the facts.  Meanwhile, those sciences that were the first to make progress have helped to develop others which, like Biology and Sociology, present greater difficulties; and it becomes more and more apparent that the distinctions drawn among sciences are entirely for the convenience of study, and that all sciences tend to merge in one universal Science of Nature.  Now, this process of the ’unification of knowledge’ is almost another name for deduction; but at the same time it depends for its reality and solidity upon a constant reference to observation and experiment.  Only a very inadequate notion of it can be given in the ensuing chapters.