While Jay Gatsby is not a total fool since he has been able to attain the materialistic American Dream and elevate himself economically and socially to a certain extent, he is, at the same time, a tragic figure because of his foolish, romantic delusion that he can regain Daisy's affections now that he is affluent.
One foolish act that Gatsby commits, for instance, occurs when he has Nick bring Daisy to his mansion and he pulls out a multitude of color shirts to impress her. Similarly, Gatsby is excessive in his material possessions in the hopes of impressing his apotheosized Daisy; for example, his car is almost mythical in appearance with its "triumphant hatboxes...with a labyrinth of windshields that mirrored a dozen suns" and "fenders spread like wings." His concoted background is likewise foolishly unrealistic as he
"live like a young rajah in all the capitals of Europe...collecting jewels, chiefly rubies, hunting big game, painting..."
Finally, the most foolish action of Jay Gatsby is his standing vigil outside the window of the Buchanans' the night of the killing of Mrytle Wilson. For, inside the house Tom and Daisy conspire against him in an act that leads to Gatsby's tragic death while he foolishly believes that Daisy feels affection for him.
Gatsby is a character who mixes intelligence in certain areas with blindness in others. He successfully reinvents himself and manages to achieve the "American Dream" of elevating himself from poverty and obscurity to a great degree of material success.
There are two main areas in which he tends to be imperceptive and let his own hopes and aspirations blind him. The first is that he does not understand the American class system and thinks that mere wealth will gain him acceptance into the old money circles exemplified by East Egg and the Buchanans. In his desire to be accepted into that society and in his inability to realize that he never will be, he is foolish.
The next area in which he is foolish is his love for Daisy, which is in many ways entangled with his social aspirations, He does not see clearly that she is beautiful but self-centered and that his romantic conception of his love for her is really an illusion. She is, in fact, married to the "right man" for her, an equally selfish member of the social elite to which she belongs by family background and ideology.