Defining terms is something that happens all the time. Whether it’s in a formal setting such as a university classroom in which a professor defines new terms formally, or when we’re with our friends and they ask us the meaning of a particular word while discussing… a book, say.

But the way in which we define something is not entirely enlightened; we tend to give synonyms. This is effective, but perhaps not the correct way to give a definition. What happens when, for example, there is no suitable synonym? And those that due render effective definitions don’t often understand the principles behind what they are doing.

So there is, in fact, a formal method and mechanism for formulating definitions. The basic stencil for a definition is this: “An A is a B which X, Y, Z”, where A is what we’re defining, B is the genus to which it belongs, and X, Y, Z, etc are differentia (sic.) that distinguish A from other entities in the genus B.

For example, let’s try to define the word “bed”. Bed belongs to the genus Furniture. Of course, it also belongs to the genus, Thing, Item, Object, Household Item, and any number of categories. But the most appropriate category is the most specific one that is widely accepted and understood. Next we differentiate Bed from other members of the genus Furniture. So a bed would then be a piece of furniture which is specifically designed for sleeping. We could have chosen other methods of differentiating bed, but the method of differentiation should be such that the fewest number of clauses needs to be used. Had we used the bed’s attribute of having pillows, this would not be enough since other pieces of furniture also typically  have pillows, and we would have been forced to do further clarification. So a bed is a piece of furniture designed specifically for sleep.

Another example: let’s define “human”. A human is many things; an animal, a mammal, a living thing, a solid object, etc. Animal, however, is the most specific genus without being too specific so as to require definition of the genus itself. Saying Mammal would have been going too far because Animal is close enough. Now there are many animals. What makes humans human? We could say that humans are animals that laugh. But then laughter is not intrinsic to humans. And if we said an animal that sneezes, then we would need other clauses as well since other animals also sneeze. A correct choice would be to say that humans are animals that are rational – humans are rational animals.