I hear a lot of drawing instructors say to "focus on the fundamentals", but I am unclear as to what the fundamentals of drawing are. For many of the other arts, the fundamentals are clear, in music, its scales/chords. Every music instructor will teach you about these things. 

In writing, you have grammar and story structure (such as the heroes journey and the three act structure), every writing instructor will teach you about these things.

However, with drawing, it seems that every art instructor talks about different things as being fundamental. 

So what are the fundamentals of drawing?

By drawing, I mean drawing realistically, from life or from imagination