The so called 4th dimension is Time. Time is the dimension of change.

(Dimensions are measurement sets.)

For example motion (changing position) is measured in, say, miles per hour. The per hour bit is time calibrating the rate of change, i.e. the rate of motion.

And you change (breathing, blood pumping etc), and everything around you changes (the earth spins). To live is to change.

So if you are alive you are changing…i.e. you are living in the 4th dimension.

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