"Motives are almost impossible to establish"

This is the point on which I partly agree and partly disagree.

I agree that:

• people conceal motives,
• governments conceal motives,
• documents are incomplete,
• some evidence is destroyed.

However, historians do not conclude that motives are unknowable.

Instead they distinguish degrees of confidence.

For example:

High confidence

Lincoln explicitly states motive X in multiple letters and speeches.

Medium confidence

Lincoln never states motive X directly, but many actions and documents point toward it.

Low confidence

The motive is inferred mainly from outcomes.

So historians usually do not say:

Motives can never be known.

They say:

Some motives are much harder to establish than others, and certainty varies depending on the evidence available.