How is the Concept of Simple Diffusion Possible
Simple diffusion is the movement of solutes from an area of high solute concentation to an area of low solute concentration. 

Here are a few ways molecules will make it through membrane into cell:

(A) Fuse with the membrane and enter due to like polarities i.e non-polar subtance fuses with non-polar membrane and passes through membrane

(B) Small polar molecules CAN move into cell but at a much slower rate

(C) Through transmembrane integral protein channels such as aquaporins, K+ leak channels, and other channels that DO NOT require energy, and are ALWAYS open

If you want to understand how it was determined which molecules were able to pass through the membrane look for some resources about Overton's Plasmolytic Method.

Alongside that the definition of the partition coefficient will give you some insight on how fast molecules will pass through membrane, if at all.

Good luck on the searching!

@ Ro Siv - Simple diffusion is not facilitated diffusion