Sol Lewitt's "Sentences on Conceptual Art"

#10 "Ideas can be works of art; they are in a chain of development that may eventually find some form. All ideas need not be made physical."

I have some issues with this statement. I don't agree with ideas being art at all. If  by ideas he means a thought, something abstract that has no physical form...I don't see how that is art unless it is executed in some way. I think for something to be art, it should have some kind of form; it doesn't necessarily need to be a physical form. But I am confused as to what he interprets as "physical" does that mean something one can touch and see? If he means that art does not need to be in a 2d or 3d form, then I would agree. I think art could be in other forms as well--sound or whatever. Could sound/video be physical? I would have a better understanding if I knew what his definition of art is, because it tends to be very subjective, what he means by ideas and what is "physical."

I also thought some of his sentences contradict each other. With more thought, I'll respond to that.