In this article we discuss one of the more famous, if not a little quirky, paradoxes in philosophy to do with how we as humans derive meaning and logic in life. It is the Sorites Paradox.

Let’s start with some thought experiments to illustrate the key point here.

This is actually a restatement a the original sorites paradox, usually attributed to the ancient Greek logician Eubulides of Miletus. ‘Sorites’ comes from the Greek word sows, meaning a ‘heap’, as the original formulation of the puzzle features a heap of sand. Expressed in terms of addition (of sand grains) rather than subtraction (of hairs), the argument looks like this:

But everybody would surely baulk at this conclusion. So what can have gone wrong?

Faced with an unpalatable conclusion of this kind, it is necessary to track back over the argument by which it has been reached. There must be something wrong with the premises on which the argument is based or some error in the reasoning. In fact, in spite of its great antiquity, there is still no clear consensus on how best to tackle this paradox, and various approaches have been taken.

One way out of the paradox is to insist, as some have done, that there is a point at which adding a grain of sand makes a difference; that there is a precise number of grains of sand that marks the boundary between a heap and a non-heap. If there is such a boundary, clearly we do not know where it is, and any proposed dividing line sounds hopelessly arbitrary: do 1001 grains, say, make a heap, but not 999? This really is a big slap in the face for common sense and our shared intuitions.

More promising is to take a closer look at a major assumption underlying the argument; the idea that the process of construction by which a non-heap becomes a heap can be fully and reductively analysed into a series of discrete grain additions. Clearly there are a number of such discrete steps, but equally clearly it seems that these steps are not fully constitutive of the overall process of heap-building.

This faulty analysis fails to recognize that the transition from non-heap to heap is a continuum, and hence that there is no precise point at which the change can be said to occur. This in turn tells us something about the whole class of terms to which the sorites paradox can be applied: not only heap and bald, but also tall, big, rich, fat and countless others. All of these terms are essentially vague, with no clear dividing line separating them from their opposites – short, small, poor, thin, etc.

One important consequence of this is that there are always borderline cases where the terms do not clearly apply. So, for instance, while there may be some people who are clearly bald and others who are clearly not, there are many in between who might, according to context and circumstances, be designated as one or the other. This inherent vagueness means that it is not always appropriate to say of a sentence such as ‘X is bald’ that it is (unequivocally) true or false; rather, there are degrees of truth. This at once creates a tension between these vague terms that occur in natural language and classical logic, which is bivalent (meaning that every proposition must be either true or false).

The concept of vagueness suggests that classical logic must be overhauled if it is to fully capture the nuances of natural language. For this reason there has been a move towards the development of fuzzy and other multivalued logics.