There is obviously some sense in which I am the same person as I was yesterday, and, to take an even more obvious example, if I simultaneously see a man and hear him speaking, there is some sense in which the I that sees is the same as the I that hears. It thus came to be thought that, when I perceive anything, there is a relation between me and the thing : I who perceive am the "subject," and the thing perceived is the "object." Unfortunately it turned out that nothing could be known about the subject : it was always perceiving other things, but could not perceive itself.
Source: Religion and Science, 1935, chapt. 5:
More info.: https://russell-j.com/beginner/RS1935_05-070.HTM