"Paragraphs on Conceptual Art"

After reading Sol Lewitt’s article “Paragraphs on Conceptual Art” I feel a little confused because he defines his work as conceptual art, and he defines the difference between perceptual art as something more visual, and the conceptual art as something in which the material or the visual solution is not important, but looking at his work, I have the sensation that the visual solution or the use of material is really important. Here probably we are dealing with conception and perception?..
Another point I don’t understand is the importance of avoiding the “physical aspect of the work” when we can see the physicality of his work and the work of other conceptual artists.