Howard Roark never attempts to argue the law, or to argue that he was legally justified in destroying Cortlandt Homes, the model housing project meant to provide cheap and durable housing for low-income renters. Instead, his defense comes from his personal philosophy of rational self-interest, and of the right of a man to own and distribute his own ideas. In his speech to the jury, he explains how he stands with egoists of history, creating without concern for the will of the collective:

"I designed Cortlandt. I gave it to you. I destroyed it.

"I destroyed it because I did not choose to let it exist. It was a double monster. In form and in implication. I had to blast both. The form was mutilated by two second-handers who assumed the right to improve upon that which they had not made and could not equal. They were permitted to do it by the general implication that the altruistic purpose of the building superseded all rights and that I had no claim to stand against it."
(Rand, The Fountainhead, Google Books)

In other words, Roark believed his claim to the design and the purpose of the homes to be superior to the claim on it by the public. His design was given in contract on the condition that the homes be built exactly as he designed; when they were altered for no reason, he refused on a moral basis to allow his design to be perverted by the will of others for any reason. Roark rejects the idea that his design is subject to alteration, especially when he sees that the alterations are done for no pragmatic purpose, but for vanity and superficiality. Again, he explicitly admits to the exact crime charged, with the caveat that it is only a crime according to the public view that no man is entitled to own his own ideas. Roark's explanation of his philosophy sways the jury and he is acquitted.