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4D (four dimensions) is an important idea in geometry.
Three-dimension space (3D) has length, depth and height. The fourth dimension is in relation to that three dimensions like a vertical dimension.
In some branches of physics time is considered as a fourth dimension but in real time people are not allowed to perceive it and it is used for modeling physical and social causes.
In mathematics, four-dimension space is an abstract concept which has been studied by mathematics and philosophers for almost three hundred years.
Mathematicians who studied four-dimension space include Möbius, Schläfi, Bernard Riemann, Charles Howard Hinton, etc.
In twentieth century the concept of spacetime was developed. It connects space and time. The difference is that spacetime is not a Euclidean space but it is called "Minkowski continuum". The four-dimension space was mostly studied in the nineteenth century.
- R. Ruyer, Paradoxy vedomi. Praha: PedF UK 1994, str. 78n