The question “what is an individual?” seems like an easy one to answer, but as will be seen the answer is more involved than would first appear to be the case. Other ways of putting the question include “what is an entity?” and “what makes an object a recognisable something?” The obvious answer is that an individual (or entity or thing) is distinct from other things, i.e. it must be isolated from other things, it must have boundaries that make it distinct from other things. Without these boundaries it would simply merge into everything around it. This is just as true of the chair I am sitting in as it is of my own body.
But nothing can be totally cut off from everything around it. This is much more obvious with our own bodies than it is with, say, a rock. For us to survive we have to take in food and excrete waste products. So we ‘merge’ with our external world in the form of our food intake and our waste products. During summer we sweat as a means to cool down. We have several layers of skin with the outer ones being constantly shed into our environment and the inner layers being constantly built up to replace those lost. These are further examples of how we are not totally separated from the world around us. Yet we can recognise each person is a distinct entity or being. We are distinct yet not distinct at the same time. We are all living contradictions!
Having looked at our whole bodies as individuals we can go further and look inside at our organs and cells. Each organ can be recognised as an individual. The heart, kidneys, lungs etc are all distinct from each other. But it is obvious that when isolated a heart, as with any other organ, cannot survive. So in one way it is an individual — it has properties and a structure that we can recognise make it a heart (it pumps blood round the body, it has special valves that other organs do not have, etc). In another way it is not an individual — it is most definitely not isolated from the world around it, it is a part that belongs to a bigger whole (i.e. our whole body).
Moving further down the scale of size we come to our cells. Again, these are obviously individuals. In fact there are many different types of individuals amongst them. Give an expert a human cell and they can tell you which part of the body it comes from. Our digestive system, for example, has many different types of cell that depend on what they are used for. Another example is our blood which contains cells for transporting oxygen around our bodies and several types of cell that help us combat diseases. Even our skin has several different types of cell. But we can still see that these cells, whilst being individuals, are parts of our whole body which is another individual. So they are distinct entities (or wholes) in one sense but in another way they are parts of another individual (or whole).
Cells are also distinct yet not distinct from the surrounding world in the way discussed with our bodies as a whole. With cells it is easier to see that there is a lot of exchange with the surrounding environment. A typical cell has a surrounding wall that keeps it separate from the world around it but at the same time allows certain molecules to come into or leave the cell. These exchanges are necessary for the cell to live. Furthermore, because the individual cell is so small it is less buffered from changes in the world around it that could harm it. For a single celled animal or bacterium in water even a slight change in, for example, salt concentration around it can cause a lot of damage to it, or even death. Compare this with a multi-celled animal such as a human in which billions of cells make up the individual. Our skin has many layers of cells with the outermost layers being dead cells and this skin acts as a buffer against some of the potentially damaging effects of our environment. Now if a human sits in a bath of salty water for quite some time the only thing noticeable is wrinkly skin. The person in the bath doesn’t even feel ill, and certainly does not die!
So far I have only used examples from animals but the argument works just as well for other living things/individuals. Plants also take in substances and excrete others. They photosynthesise, which means they absorb light from their surroundings and use the energy from it to make chemicals that help to keep them alive. This brings us to another way that living individuals merge with their surroundings. Every living thing absorbs energy in some form and also ‘leaks’ energy into its surroundings. In the example of photosynthesis plants absorb light energy. Mammals, including ourselves, ‘leak’ heat to our surroundings.
This idea that the individual is both separate and not separate from its surroundings also applies to non-living individuals. A rock would appear at first to be completely separate from its surroundings. But quite quickly we can show that it is not. Rocks will often absorb different chemicals (exactly which chemicals and to what degree depends on the type of rock). It is not uncommon for a rock to absorb water or to be worn down over time by dripping water, rain, or waves.
On a topic related to rocks, consider a mountain as an individual/entity. The same arguments for leaking into and out of a rock apply to mountains. Of more interest with mountains-as-individuals and the question of how separate they are from their surroundings is the question, where does a mountain begin? The top of a mountain is easy to find but where does the base of the mountain begin? The largest mountain in the world, Mount Everest, has such gentle slopes at its base that it is difficult to tell exactly where the actual mountain begins. So at its base this mountain has a fuzzy boundary with the world around it. So even something as big as Mount Everest is distinct but not distinct from its surroundings!
I could use further examples but my major point has been made. Throughout these examples I have shown how something can be both separate and not separate from its surroundings at the same time. It is an example of what Marxists call a dialectical contradiction. It is not that there is something wrong with the way we are interpreting the world that leads us into this contradiction. It is simply that the world/universe actually is contradictory in certain special ways, and the nature of the individual being both separate and not separate from its surroundings is one of those contradictions.