Entia non sunt multiplicanda praeter necessitatem

Entities must not be multiplied beyond necessity.

3 is a prime, 5 is a prime, and 7 is a prime. Why bother with non-prime numbers when the primes can do everything?

Never increase, beyond what is necessary, the number of entities required to explain anything.

The explanation requiring the fewest assumptions is most likely to be correct.

When you have two competing theories that make exactly the same predictions, the simpler one is the better.

It is vain to do with more what can be done with less.