Why is 1 neither a prime or a composite?

Answer 1

The concept of prime or composite is defined only for whole numbers which are greater than 1.

One reason for this is to ensure the fundamental theorem of arithmetic remains true. In simple terms, this states that any positive integer which is greater than 1 can be expressed as a multiple of a unique set of prime factors.

If 1 were allowed, then 6 for example, could be expressed as 2*3 or 1*2*3 or 1*1*1*2*3 and the uniqueness no longer applies.