“The evidence does not merely derive from the same assumption”

Suppose you argue:

Theory: capitalism expands when labor becomes mobile.

Then you say:

“We know labor became mobile because capitalism expanded.”

That is circular.

You are using the conclusion to prove the premise.

Instead, use evidence independent of the conclusion:

Census data show workers moving from rural areas to industrial cities.

Now you have evidence of mobility independently of the proposition that capitalism expanded.