Numbers may constitute a sixth sense and serve as the ultimate basis for abstract and symbolic thinking.

Numerosity is the appreciation of numbers with two core processes: subitization and fuzzy set comparisons.

Subitization is the sudden recognition and differentiation among one, two, and three things.

Human infants and monkeys can subitize but the limit appears to be three things.

Traditionally, humans are said to have five senses: vision, hearing, taste, smell, and touch. But what if there is a sixth sense? Although extrasensory perception (ESP) is often proffered as the sixth sense, the actual consistent empirical evidence is so weak or non-existent as to make that supposition highly questionable. However, what if a sixth sense lies not in ESP or some mystical perception, but in something far more fundamental and demonstrable: humans’ innate sense of numbers?

What Makes a Sense Primary?

First, what qualifies a sense as primary? The traditional senses share a key feature: They are hardwired. People don’t teach their children what sweetness is or how to distinguish red from blue: We simply give names to experiences we already possess. Dedicated brain neurons process human senses automatically, innately, and without learning or instruction.

I argue our sense of numbers works the same way. We don’t truly teach children to recognize “one,” “two,” or “three” things: We are simply........

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