The Outsider is an enduring classic of existential writing by Albert Camus
'Mother died today.
Or maybe yesterday, I don't know'
Meursault is different. He will not lie. He will not pretend.
He is true to himself.
So when his mother dies and he is unmoved, he refuses to do the proper thing and grieve. Returning to Algiers after the funeral, he carries on life as usual until he becomes involved in a violent murder.
In court, it is clear that Meursault's guilt or innocence will not be determined by what he did or did not do.
He is on trial for being different - an outsider.