This assumption is reasonable. Its idea is that there is some connection between Y and X, no matter how complicated this connection is, let y=xt, T is the variable of the connection between X and Y, and this T is the intermediate variable of X and Y, which is their connection. For example:

3x+4y- 1=0, we can set y=xt, that is 3x+4xt- 1=0, that is, x= 1/(3+4t).

... 1

And x=y/t, so 3y/t+4y= 1, that is

y= 1/(3/t+4)

....2

The two equations, 1 and 2, can eliminate t, which will be equal to the original equation, so t is only an intermediate variable and only temporarily borrowed.