People believe that perception is reliable and that what they perceive reflects objective reality. On this view, we perceive a red circle because there is something out there that is a red circle. It is also commonly believed that perceptual reliability is threatened if what we see is allowed to be influenced by what we know or expect. I argue that although human perception is often (but not always) highly consistent and stable, it is difficult to evaluate its reliability because when it comes to perception, it is unclear how one could establish a fact of the matter. An alternative to thinking of perception as being in the business of truth, is thinking of it as being in the business of transducing sensory energy into a form useful for guiding adaptive behavior. On this position, perception ought to be (and, as I argue, is) richly influenced by some types of knowledge insofar as this knowledge can aid in the construction of useful representations from sensory input.