One of the simplest and most effective ways to achieve an effect of perspective drawing is to employ the technique of relative size, or subjects that diminish in size the further away they are from the observer. This is something we are all familiar with, and that even children learn pretty early on to use in their own simple drawings.
It works like this. When we look at the world around us, we judge the relative size of the objects we see or similar objects in terms of our prior or present experience of them.
You are sitting in an armchair looking at the cat and thinking you would like to pick him up. Since he is a cat, as soon as you bend over with your arms outstretched, he is going to yawn, stretch and start walking away from you with a snap of this tale and a backward slant to the ears. If you are allergic to felines he will do the contrary, and jump up on your lap and adore you, but that's another story.
As you look at the cat walking away, you get an image on your retina that looks something like this:
And you think to yourself, there's the tail end of that cat. Then you get another retinal image that looks like this:
Since it looks familiar, you say to yourself, gee, that smaller, but is the same cat.
And then you get this on your retina:
And you think, that's even smaller but it's still that same stupid cat that I didn't want to pick up anyway since it would have got hair all over me.
The fact of you knowing that they all are the same cat is referred to as size constancy. Further, you interpret the series of three smaller and smaller images as meaning that the cat is moving further and further away from you into the distance.
This is why, as well, if you see a very small cat, especially compared to a very large dog…
You will interpret the Is being distant (and of little interest to you) and the dog is being near (and of some concern to you) because of their relative size. And as we mentioned, this principle is so much a part of everyday experience that even children adopt technique early on.
Just goes to show you that perspective can be child's play!