Is but a preposition

Answer 1

The word BUT can "act" as a preposition but is typically a conjunction and possibly an adverb.

In the sentence "No one can help him but himself" the phrase "but himself" is actually an elliptical (truncated) form of "but he can help himself" where BUT is a conjunction. Here the word BUT functions like the word except, which is more consistently defined as a preposition.

Answer 2

The word "but" can function as a conjunction, coordinating conjunction, preposition, or adverb in a sentence depending on context. As a preposition, it can indicate an exception or exclusion in a sentence. For example: "All but her friends attended the party."