  Before this period, optical illusions were understood as just that, errors in judgment, quirks of the brain. The sense organs were seen as simple media with a direct link to the brain. Their only role was to transmit a sensation to the brain, where it was then interpreted. Mach argued that the processing does not all occur in the brain. The quirks of judgment are not due to processing errors in the brain, but innate features of the senses themselves.