They’re called the ‘exteroceptive’ senses because they carry information about the external world. But your body also has receptors for events occurring inside you, such as your beating heart, expanding lungs, gurgling stomach and many other movements that you’re completely unaware of. They’re traditionally grouped together as another sense, called ‘interoception’.

Yet a proper answer to this question is even more complex and interesting. For one thing, your body has receptors to carry other types of information, such as temperature, that we don’t usually consider to be senses. Also, some of your receptors are used for more than one sense. Your retinas, for example, are portals for the light waves you need for vision, but some retinal cells also inform your brain if it’s daytime or nighttime.

This unnamed ‘day/night sense’ is the basis for circadian rhythms that affect your metabolism and your sleep/wake cycle. Even senses that seem fundamental, such as vision, are intimately entwined with other senses that seem separate. For example, it turns out that what you see, and how you see it, is yoked to your brain’s tracking of your heartbeat, which is part of interoception.