Actually I am reading from Einstein's book of which one of the chapters is titled "Minkowski’s Four-Dimensional Space".

It is a long way ahead of my learning but a part of it intrigues me and and would appreciate perhaps a bit of help so that I cold perhaps understand exactly (or maybe just more precisely ) what he (Einstein ) is alluding to.

This is the passage (and also the link - Chapter 17. Minkowski’s Four-Dimensional Space. Einstein, Albert. 1920. Relativity: The Special and General Theory)

In order to give due prominence to this relationship, however, we must replace the usual time co-ordinatetby animaginary magnitude

ct proportional to it. Under these conditions, the natural laws satisfying the demands of the (special) theory of relativity assume mathematical forms, in which the time co-ordinate plays exactly the same rôle as the three space co-ordinates. Formally, these four co-ordinates correspond exactly to the three space co-ordinates in Euclidean geometry. It must be clear even to the non-mathematician that, as a consequence of this purely formal addition to our knowledge, the theory perforce gained clearness in no mean measure.

So where could I find this result explained in detail (or the mathematics immediately relevant to it) ?

Or do I have to start from scratch really and study it properly (perhaps beyond me)?

Would I be right to think that for starters it seems to explain the negative sign for (ct)^2 in the the metric for flat Minkowski space-time (since ^2 =-1 )

(I had naively assumed it would be positive ,the same as for the x, y, and z )

To explain how I came to be (trying to ) reading that part of Einstein's Paper I googled "is spacetime real" (in quotes) which brought me to an old discussion on the Physics Forums (Spacetime doesn't really exist does it?) where this Paper of Einstein's was referred to.