ONE way of estimating the validity of a critic's judgments is that of studying his mental peculiarities as generally displayed. If he betrays idiosyncrasies of thought in his writings at large, it may be inferred that these idiosyncrasies possibly, if not probably, give a character to the verdicts he passes upon the productions of others. I am led to make this remark by considering the probable connection between Professor Tait's habit of mind as otherwise shown and as shown in the opinion he has tacitly expressed respecting the formula of evolution.
Daily carrying on experimental researches, Professor Tait is profoundly impressed with the supreme value of the experimental method; and has reached the conviction that by it alone can any physical knowledge be gained. Though he calls the ultimate truths of physics "axioms," yet, not very consistently, he alleges that only by observation and experiment can these "axioms" be known as such. Passing over this inconsistency, however, we have here to note the implied proposition that, where no observation or experiment is possible, no physical truth can be established; and, indeed, that in the absence of any possibility of experiment or observation there is no basis for any physical belief at all. Now, "The Unseen Universe," a work written by him in conjunction with Professor Balfour Stewart, contains an elaborate argument concerning the relations between the universe which is visible to us and an invisible universe. This argument, carried on in pursuance of physical laws established by converse with the universe we know, extends them to the universe we do not know: the law of the conservation of energy, for example, being regarded as common to the two, and the principle of continuity, which is traced among perceptible phenomena, being assumed to hold likewise of the imperceptible. On the strength of these reasonings, conclusions are drawn which are considered as at least probable: support is found for certain theological beliefs. Now, clearly, the relation between the seen and the unseen universes can not be the subject of any observation or experiment; since, by the definition of it, one term of the relation is absent. If we have, then, no warrant for asserting a physical axiom save as a generalization of results of experiments—if, consequently, where no observation or experiment is possible, reasoning after physical methods can have no place—then there can be no basis for any conclusion respecting the physical relations of the seen and the unseen universes. Not so, however, concludes Professor Tait. He thinks that, while no validity can be claimed for our judgments respecting