You would think you knew the human body pretty well.  You’ve seen it all your life, dressed and undressed.  Pictures everywhere—great paintings, cartoons, erotic magazines.  But start drawing one and find out what you know—see how little. Only drawing will teach you not only to get it right but to see it right.  Drawing is seeing. Until you have to put down what you know on paper, you will only be fooling yourself that you know.  Hic Rhodas, hic salta.
Why are we so ignorant?

The mind isn´t too demanding.  It has never paid too much attention to the information eyes report to it. Often it will be satisfied with a little conventional sign—it isn´t necessary to copy things exactly—a square will suffice for a window, a circle will do for a head.  The mind knows what you mean.  Words would serve just as well.

If you want to draw, to copy what you actually see, you must look more closely than you ever did.  Remember that though you have often had things in front of your eyes, you have not looked at them.  To know them you shouldn´t consult your mind, which has a big mouth but knows nothing about the matter.  Your mind will want you to simplify as you work and to make the image homey and familiar.  You have to resist that advice.  Turn off your mind.  It takes practice but you must draw only what you see.
Then, once you have slaved copying exactly what you see, try to draw it from memory.
That’ll teach you.