What scientific principle does the Time Traveller try to convince the others to accept?

I think that this question is asking about the dialogue sequence that happens in the opening chapter of The Time Machine. The Time Traveller is attempting to get the other people in the room to accept the idea that anything that exists, exists in four dimensions. The Time Traveller calls the four dimensions "directions," but it is the same scientific concept. An object has length, width, height, and duration. The Time Traveller goes on to explain that space and time should not be thought of as separated from each other. He explains that time is related to a three dimensional world. It isn't separate from space. The Time Traveller doesn't use Einstein's "spacetime" vocabulary, but it is clear that Wells is expressing the same concept.

"Scientific people," proceeded the Time Traveller, after the pause required for the proper assimilation of this, "know very well that Time is only a kind of Space."

He explains time this way in order to get the men thinking about the possibility of moving through time in the same way that they move through space.

Long ago I had a vague inkling of a machine
[...]
That shall travel indifferently in any direction of Space and Time, as the driver determines.