It is along the same straight line as the other

Question

Thus we come to know much more about the relations of distances in physical space than about the distances themselves; we may know that one distance is greater than another, or that it is along the same straight line as the other, but we cannot have that immediate acquaintance with physical distances that we have with distances in our private spaces, or with colours or sounds or other sense-data.

[Problems of Philosophy, Bertrand Russell, Chapter III]

I can't imagine what Russell said in the bold text. Could you explain this to me? I think he meant that one distance is parallel to another distance. Is it right?

Answer 1

Russell is saying that if you drew a line between yourself and some object (call it line AB), and then another object farther away (call it line AC), you might easily recognize that all three points ABC are on the same line, and also that the second is of greater distance from you than the first. However your understanding of distance is relative -- but there is no abstract understanding of the distance itself except as measured relative to other things.

To put it another way: You know that AC is longer than AB, and that, from your point of view, they lie on a straight line relative to each other. But in order to actually measure AC you have to compare that distance to something else. For example, suppose that object C is ten times farther away from you than object B. If we call distance AB "one meter" than we can say AC is "ten meters" away.

Russell compares these perceptions of "physical space" with other sense data like hearing or smell, in which the sensations are immediate and entirely contained in our "private space".