What does 'at its simplest' mean?
'You mean how did it take her? Oh, a complete indifference to everything but the thing she wanted at the moment.'
'That isn't madness; that is merely the criminal mind at its simplest.'

Does 'at its simplest mean' 1) in fact or 2) potential?

Question

What does 'at its simplest' mean?
'You mean how did it take her? Oh, a complete indifference to everything but the thing she wanted at the moment.'
'That isn't madness; that is merely the criminal mind at its simplest.'

Does 'at its simplest mean' 1) in fact or 2) potential?

Answer

It's in fact, at least as perceived by the speaker.The phrase means  "in its simplest form" or "in essence", i.e. that she was very focused on what she wanted to the degree she neglected everything else, which the speaker imputes to a criminal nature.