As we are beings of the third dimension we can both see and understand the 0th dimension, 1st dimension, and 2nd dimension. We can understand the 3rd dimension, in which we live but cannot see it, as we can the other lower dimensions. And as we are relative to the third dimension we cannot, in theory, conceive of a dimension beyond that which is three-dimensional.

It has been speculated that this 4th dimension is time in that all things in our world are either restricted by time or functions of it. However, in order to both see and understand the 3rd dimension we would have to no longer be restricted by time (for it is time that restricts us from being at both one point in space and another simultaneously).

Without the restriction of time we could literally be in all places at once, and that would not be time but more the absence of time, or eternity. Eternity is something we cannot comprehend as humans because we operate inside of time, just like the creatures in flatland could not comprehend height and depth, so we cannot comprehend eternity, the fourth dimension.