On the philosophical underpinning of his scientific approach.

'My affirmation that the difference between sensory experience and hallucination is a purely hypothetical (conventional) difference belonging to one of those categories invented by us without independent logical content – justifiable only through expediency: this is a mere cliche. I have mentioned it in order to show that the free selection of conceptually constructive elements, which are freely imposed and not empirically deducible, does not begin in the actual scientific domain but belongs to everyday thought'. The letter also responds to a question from Besso about the non-linearity of equations for the explanation of reciprocal effects, providing an example of the way in which field theory replaces the notion of force, and attempts to grasp a problem expounded by Besso's son, Vero (but poorly explained by Besso) about a schema for distinguishing between similar objects.