Plurality must never be posited without necessity.
Numquam ponenda est pluralitas sine necessitate.
William of Ockham, Commentary on the Sentences (Ordinatio) I, dist. 27, q. 2.
The Franciscan friar Ockham uses this principle as a measure in his arguments.
If two causes are enough to explain a thing, putting forward a third does not add to knowledge; it adds to the crowd.
The principle later came to be called a razor, because it shaves away the excess.
It should not be taken as a sure measure of truth; it is a discipline of thought.
The longer an explanation grows, the more one believes one has understood; often one has only piled up assumptions.