When we think about how we can move through the Universe, we immediately think of three different directions. Left-or-right, forwards-or-backwards, and upwards-or-downwards: the three independent directions of a Cartesian grid. All three of those count as dimensions, and specifically, as spatial dimensions. But we commonly talk about a fourth dimension of a very different type: time. But what makes time a dimension at all? That’s this week’s Ask Ethan question from Thomas Anderson, who wants to know: I have always been a little perplexed about the continuum of 3+1 dimensional Space-time. Why is it always 3 [spatial] dimensions plus Time? Let’s start by looking at the three dimensions of space