We have seen that one whole series of changes has been unhesitatingly described as a series of changes in my sensations.

Why was this?
Because it was observed to depend upon changes in the relations of my body, my senses (a certain group of experiences), to the bit of experience I call the fire.

Another series was described as a series of changes in the fire.

Why?
Because, the relation to my senses remaining unchanged, changes still took place, and had to be accounted for in other ways.
It is a matter of common knowledge that they can be accounted for in other ways.

This is not a discovery of the philosopher.