According to Aristotle, what is an "artless" proof?

A proof that involves no art

A proof that is obvious

A physical or scientific proof

A well-composed argument

Ah Aristotle! A fine philosopher for some and a charlatan for many others.

In this case, the answer you are looking for is A physical or scientific proof.

This is a piece of information containing facts or with logical proof. There is no invention required on the part of the speaker and no need to mess with the information. The information from the speaker is preexisting and accepted.

You can compare this with artistic proof where the reader creates and states information of their own making. The concept does not exist already and the speaker is actively creating it with their words.