External perception is the faculty by which the external world becomes known to us. The external world begins where the world of consciousness ends.

In order for external perception to take place, three conditions must be met:

1. Some object must be in our vicinity. This seems obvious, but perception sometimes occurs even in the absence of an object. This is called a hallucination.

2. Certain physiological conditions must be fulfilled. Again, there are three: The object must come to the attention of a sensory organ; the nervous stimuli generated must be transmitted through the body; and this transmission must reach the brain.

3. The self must intervene. Sensory stimuli are multiple and diverse, and unified perceptions occur only through the intervention of the self.

Of these three conditions, here we'll study only one – the relationship between the object and the senses. We won't concern ourselves with the existence of the object or the intervention of the self. Instead, we'll focus on the organs – called senses – that mediate between objects and the brain.

Five senses are typically recognized: touch, smell, taste, sight, and hearing. But it's important not to restrict the word “senses” just to the sensory organs that are the intermediaries between the external world and the self. We define the word more broadly as “sources of information about the external world” because there are some senses that aren't localized. Two senses – known to us only recently – are not associated with a specific organ.