Motive vs. Intent, and detecting design

There is an interesting discussion going on about "How do you prove purpose", Helena Petrovna Blavatsky lead blogger, at Overwhelming Evidence.

Here was my contribution:

One question that commonly arises when people discuss design in the universe is "how can you tell it is design if you do not know the motive of the designer?" Or perhaps the "purpose" of the designer?

Actually you can. The police do it every day in criminal investigations.

For one thing, there is a difference between motive and intent. Confusion on that subject can sometimes result in confusion about detecting design.

Legal cases typically turn on intent, not motive.

Here is an example: Harry and Jack are having a somewhat tense conversation over a beer and steak at the local pub.

Harry seizes a steak knife and tries to plunge it into Jack's ribs.

Pub regulars overpower him and the police are called. He is charged with assault with a deadly weapon.

That's intent.

No one knows Harry’s motive, but no one needs to know his motive. What he intended was obvious. And it is a crime.

A later investigation may turn up a motive - perhaps Jack had informed Harry that ...

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Also, why do people ignore lessons from human evolution when it suits us?