well from wat my teacher xplaind... the difference between clasical, modern and post modern art is this. (i'l just include clasical so u can understand it better) clasical artists bliv dat beauty is universal. if a certain thing is beautiful, it is still beautful yesterday, tommorow and for the next hundred yrs to come, in mars, jupiter or wat ever planet. so they made standards for beauty and folowd it to make beautiful architecture or things (like the fibunacci number). dat idea still remains until the modern period. but instead, modern artist bliv that every person can tell wether a thing is beautiful if he/she looks at it with disinterest, meaning stripping down all biases (culture, experience..etc). these biases can affect our judgement of beauty that is why a thing may not be beautiful for the other and may be to another. that is why modern abstract composition remains btiful even if you cant understand the figures. while post modern art still blivs in the idea of the two periods but they focus more on the things that made us see beauty in diff. ways. our biases, our culture, xperience, etc made us beautiful. they are now concernd with individuality of the artists or of a certain piece. if it is percieved ugly becoz its diferent, then for the post modern artists, it is beautiful becoz it is diferent. i hope this answer helps you...sorry for the spelling thou, i get tired typing