All sensations, such as sight, smell, or sound, relied on these particles. While the atoms that were emitted did not have the qualities that the senses were perceiving, the manner in which they were emitted caused the observer to experience those sensations, e.g. red particles were not themselves red but were emitted in a manner that caused the viewer to experience the color red. The atoms are not perceived individually, but rather as a continuous sensation because of how quickly they move. Epicurean philosophy employs an empirical epistemology. The Epicureans believed that all sense perceptions were true, and that errors arise in how we judge those perceptions. When we form judgments about things ("hupolepsis"), they can be verified and corrected through further sensory information. For example, if someone sees a tower from far away that appears to be round, and upon approaching the tower they see that it is actually square, they would come to realize that their original judgement was wrong and correct their wrong opinion. Epicurus is said to have proposed three criteria of truth: sensations ("aisthêsis"), preconceptions ("prolepsis"), and feelings ("pathê"). A fourth criterion called "presentational applications of the mind" ("phantastikai epibolai tês dianoias") was said to have been added by later Epicureans. These criteria formed the method through which Epicureans thought we gained knowledge. Since Epicureans thought that sensations could not deceive, sensations are the first and main criterion of truth for Epicureans. Even in cases where sensory input seems to mislead, the input itself is true and the error arises from our judgments about the input. For example, when one places a straight oar in the water, it appears bent. The Epicurean would argue that image of the oar, that is the atoms traveling from the oar to the observer's eyes, have been shifted and thus really do arrive at the observer's eyes in the shape of a bent oar. The observer makes the error in assuming that the image he or she receives correctly represents the oar and has not been distorted in some way. In order to not make erroneous judgments about perceivable things and instead verify one's judgment, Epicureans believed that one needed to obtain "clear vision" ("enargeia") of the perceivable thing by closer examination. This acted as a justification for one's judgements about the thing being perceived. "Enargeia" is characterized as sensation of an object that has been unchanged by judgments or opinions and is a clear and direct perception of that object.