On falling apples and whether tomatoes really are fruit

Imagine a film of an apple accelerating toward the ground under the influence of gravity. If we played this film backwards, what would we see? It may seem odd, but a physicist will tell us that when the film is played backwards, the apple will continue to accelerate downwards! Regardless of whether the film is played forwards or backwards, the apple will accelerate downwards.

It is because physicists use the word accelerate with a slightly different meaning to the one in everyday use that an otherwise trivial fact about the universe can be cast as such a curious statement.

So what's going on? In the everyday sense, saying that an apple is accelerating downwards implies that it is also moving downwards, but the physicist's definition does not imply this. To the physicist, an apple that is being slowed by the pull of gravity as it is moving upwards, as it appears in the reversed film, is accelerating towards the ground just as surely as it is when the film is played forwards. The physicist takes accelerating downwards and decelerating upwards to be equivalent descriptions.

The advantage of the physicist's definition is that it succeeds in unifying these two descriptions under one concept allowing for a more general mathematical framework to describe the influence of a force like gravity on the motion of objects. On the other hand, when we apply the everyday sense of the word to say that an apple is accelerating towards something, it neatly implies that it is also moving in that direction so we don't have to specify this separately. The definitions of the physicist and of the lay person differ, but there isn't any objective sense in which the former is more correct than the latter. There are simply different conventions that apply in different contexts. In this case, one is a technical term and the other is not.

The same considerations should be borne in mind when claims are made about tomatoes being fruit, the Sun being a star, black not being a color, thumbs not being fingers, and so on. In each case, the person advertising their cleverness with these claims is really just asserting a preference for one convention over another, often in contexts where it is inappropriate to do so. In the case of fruit, the botanist's definition will include any seed-bearing part of a plant. This will include tomatoes, but also things like pumpkins, zucchinis, peas, and even nuts. It's a term that is appropriate for discussions of things like plant reproduction and will also include many things that are not generally used as a food source by humans. But there is also a separate and very well-attested convention of applying the word fruit with a meaning that is sensitive to things like sweetness, fleshiness, whether they can be eaten raw and other factors that are relevant to its role as a food source, and this category has a much narrower membership.

The only standard by which we can judge the use of a word to be correct or not is whether it conforms to the conventions used by members of the speech community in the particular context in which it is being used. Hence, if a large number of people are all using a word in the same way, the particular convention they are applying can never be considered incorrect. This is true no matter how annoying, illogical, ugly or offensive this convention may be. They could be using a word in a way that is no longer consistent with its etymological roots, with a meaning that deviates from usage at some idealized literary epoch, or in any number of other ways that draw scorn from various self-appointed guardians of the English language who like to express their particular prejudices by appealing to rules that sound as though they derive from some sort of authority. But if speakers are successfully applying a convention that is genuinely attested within their speech community, there is no other standard by which their usage can be judged to be correct. Indeed, the role of dictionaries is to describe rather than prescribe these patterns of usage, and they are duly revised when new conventions are detected within the speech community. These revisions are often met with vocal protest from people who simply fail to understand this fact, and who have, since ancient times, mistaken language change for decaying standards.