… the second figure reads:

Anything of the nature of M would have the character {p}, taken haphazard,
S has the character {p};
.·. Provisionally, we may suppose S to be of the nature of M.

Still more convenient is the following conditional form of statement:

If {m} were true, {p}, {p}’, {p}” would follow as miscellaneous consequences -
But {p}, {p}’, {p}” are in fact true;
.·. Provisionally, we may suppose that {m} is true.

This kind of reasoning is very often called adopting a hypothesis for the sake of its explanation of known facts.