perceived forces, save as experimentally justified, some validity can be claimed for our judgments respecting unperceived forces, where no experimental justification is possible.
The peculiarity thus exhibited in Professor Tait's general thinking is exhibited also in some of his thinking on those special topics with which he is directly concerned as a Professor of Physics. An instance was given by Professor Clerk-Maxwell when reviewing, in "Nature," for July 3, 1879, the new edition (1879) of Thomson and Tait's "Treatise on Natural Philosophy." Professor Clerk-Maxwell writes: "Again, at page 222, the capacity of the student is called upon to accept the following statement: 'Matter has an innate power of resisting external influences, so that every body, as far as it can, remains at rest or moves uniformly in a straight line.' Is it a fact that 'matter' has any power, either innate or acquired, of resisting external influences?" And, to Professor Clerk-Maxwell's question thus put, the answer of one not having a like mental peculiarity with Professor Tait must surely be—No.
But the most remarkable example of Professor Tait's mode of thought, as exhibited in his own department, is contained in a lecture which he gave at Glasgow when the British Association last met there (see "Nature," September 21, 1876)—a lecture given for the purpose of dispelling certain erroneous conceptions of force commonly entertained. Asking how the word force "is to be correctly used," he says: "Here we can not but consult Newton. The sense in which he uses the word 'force,' and therefore the sense in which we must continue to use it if we desire to avoid intellectual confusion, will appear clearly from a brief consideration of his simple statement of the laws of motion. The first of these laws is: Every body continues in its state of rest or of uniform motion in a straight line, except in so far as it is compelled by impressed forces to change that state." Thus Professor Tait quotes, and fully approves, that conception of force which regards it as something which changes the state of a body. Later on in the course of his lecture, after variously setting forth his views of how force is rightly to be conceived, he says, "Force is the rate at which an agent does work per unit of length." Now let us compare these two definitions of force. It is first, on the authority of Newton emphatically endorsed, said to be that which changes the state of a body. Then it is said to be the rate at which an agent does work (doing work being equivalent to changing a body's state). In the one case, therefore, force itself is the agent which does the work or changes the state; in the other case, force is the rate at which some other agent does the work or changes the state. How are these statements to be reconciled? Otherwise put, the difficulty stands thus: force is that which changes the state of a body; force is a rate, and a rate is a relation (as between time and distance, interest and capital); therefore a relation changes the state of a body. A relation is no longer a nexus among phenomena, but becomes a producer of phenomena. Whether Professor Tait succeeded
