What is "direct" evidence? Is there indirect evidence?

Yes.

There is a distinction.

Direct evidence

Evidence that explicitly addresses the proposition.

Example:

A letter from a politician saying:

"We must abolish slavery because industry requires free labor."

That directly supports your claim.

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Indirect evidence

Evidence from which an inference is drawn.

Example:

• industrialists funded abolitionist politicians,
• those industrialists later benefited economically,
• free labor expanded afterward.

None of these explicitly states motive, but together they may support an inference.

Historians use both kinds of evidence, but they usually distinguish clearly between them.