If you want to get that reductive, ultimately everything in the universe is just particles and forces and interactions between them, and any concepts beyond that are arbitrary lines drawn by us humans in an attempt to organize the chaos.
In the case of senses, it's about perception. We perceive the signals from our retinas differently than the ones from our fingertips, so we give them different names (sight vs. touch).
There are also differences in the "hardware" -- sight and touch signals start with different kinds of receptors, travel down different nerves, and are processed in different parts of the brain -- but the hardware differences don't map 1:1 to the way that we categorize senses. For example, pain and pressure are processed by entirely different types of receptors, but we still call them both touch. On the flip side, balance and hearing share a lot of the same hardware, but we consider them to be different senses.
In the case of senses, it's about perception. We perceive the signals from our retinas differently than the ones from our fingertips, so we give them different names (sight vs. touch).
There are also differences in the "hardware" -- sight and touch signals start with different kinds of receptors, travel down different nerves, and are processed in different parts of the brain -- but the hardware differences don't map 1:1 to the way that we categorize senses. For example, pain and pressure are processed by entirely different types of receptors, but we still call them both touch. On the flip side, balance and hearing share a lot of the same hardware, but we consider them to be different senses.