This book advances the thesis that Relativity rejects the notion of physical substance and yet allows for the existence of substantival entities of which an event is one. The opposition between the noun substance and the adjective substantival is introduced in order to avoid confusion resulting from the use of the term substance in its historically different senses. A substantival entity corresponds to the basic meaning of substance, derived from Aristotle, which is to the effect that substance which is referred to by the subject can never be denoted by the predicate of a proposition. For example, in saying “this is an outrage,” the word “this” stands for a substance because there is no proposition of the form “the outrage is a this” which would make sense. Since the word “this” can designate any event, it is obvious that in this sense of “substance” there are substances in the world.

The various historically important meanings of “substance” came out from different attempts to answer the question : What is the nature of a thing which prevents it from figuring in a proposition as a predicate? In the philosophy of the continental rationalists it was taken for granted that a predicate is an “accident,” the existence of which depends on the existence of the subject, from which it follows by opposition that the true subject or the substance cannot become a predicate because it has an independent or autonomous existence. The same conception of predicates as “accidents” which must occur to something led the British empiricists into the definition of substance as the bearer of the properties denoted by predicates, i.e. as the substratum underlying the display of properties. Whether it is an autonomous being or a substratum, substance would have to be construed as being more essential than its “accidents” or appearances. It was only natural to conclude that if the “accidents” were really not essential to a thing, this thing might have different accidents without ceasing to be the same substance. This conclusion is expressed by defining substance as the thing which remains the same through the change of its “accidents.” Substance in this sense is objectionable because of the obvious principle that if something changes it cannot remain the same.

A defence of the notion of substance as a thing which is the same in spite of change is found in the position of classical physicists and of many philosophers. There are at least two philosophical arguments in favour of substance. First, something can change and yet remain the same if the sense in which it changes is different from the sense in which it remains the same. Since change is a replacement of “accidents,” it does not affect the essential nature of the substance. Thus substance changes superficially but remains the same essentially. This argument is obviously unconvincing because it severs an essence from its “accidents” to the extent of making unintelligible what is meant by saying that the accidents belong to the substance or that the substance owns its properties. If the substance S is indifferent to the change from the appearance A to the appearance B, there is no reason whatsoever for attributing either of these appearances to the substance S rather than to any other substance SI. The second argument relies on a linguistic consideration. Take the sentence “My car was brand-new in 1934, but it is one year old now.” This happens to be true (at the moment of writing the sentence) of my own car, and it would seem that the italicized words of the sentence refer to the same thing, so that it would seem that it is possible to have the same thing replacing its property “brand-new” by the property “one year old.” But a careful analysis of sentences like the one under consideration, which was given first by Bertrand Russell, shows beyond any ...