People with blindsight are usually not aware that they are reacting to visual sources, and instead just unconsciously adapt their behaviour to the stimulus.

A sense is a physiological capacity of organisms that provides data for perception.

The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology (or cognitive science), and philosophy of perception.

The mind considered by itself is seen as the principal gateway to a different spectrum of phenomena that differ from the physical sense data.

This way of viewing the human sense system indicates the importance of internal sources of sensation and perception that complements our experience of the external world.

Some argue that stereopsis, the perception of depth using both eyes, also constitutes a sense, but it is generally regarded as a cognitive (that is, post-sensory) function of the visual cortex of the brain where patterns and objects in images are recognized and interpreted based on previously learned information. Temporary or permanent blindness can be caused by poisons or medications.

People who are blind from degradation or damage to the visual cortex, but still have functional eyes, are actually capable of some level of vision and reaction to visual stimuli but not a conscious perception; this is known as blindsight.

Rods are very sensitive to light, but do not distinguish colors.

Cones distinguish colors, but are less sensitive to dim light.

There is some disagreement as to whether this constitutes one, two or three senses.

Neuroanatomists generally regard it as two senses, given that different receptors are responsible for the perception of color and brightness. Blindness may result from damage to the eyeball, especially to the retina, damage to the optic nerve that connects each eye to the brain, and/or from stroke (infarcts in the brain).

the different chemoreceptors for detecting salt and carbon dioxide concentrations in the blood).