One hugely important story about early modernity is the rise of the significance of the individual. Individuality is so fundamental to our contemporary way of understanding the world that it is hard to believe that people didn't always think like this. Tied to this is the idea of freedom - Americans, particularly, have a hard time imagining that the two are different, or that it is possible to have freedom without individuality, or individuality without freedom.
What is individualism? It means several things, and has several sources.
1. One of the first sources of individualism is in the late mediaeval battle between realism and nominalism. Basically, some people believed that universals existed apart from specific instances. For example, when we see many examples of chairs, we use the same word, "Chair", for all of them. That word is common to them all. Now, is there something that exists that is shared by all those things? If not, how do we know when we come to new examples of chairs that they really are chairs? Realists believed that there really was a thing called a "Chair" apart from all the concrete instances of chairs. Nominalists, on the other hand, believed that there was no such thing, that all that existed were the concrete instances of chairs.
2. As the world becomes more populated and more complex, people have to deal with others who they do not know. In the modern world, individualism becomes a kind of abstraction, in which we assume that people's actual characteristics are irrelevant to how they are treated before the law or by a state. Individualism, then, means both a minimum standard of treatment, and a maximum level of detail that can matter about someone. So, every individual has a set of rights by virtue of being a human individual (and others by virtue of being a citizen of a country, and others for other reasons). But those rights cannot take into account specifics about the individual such as hair colour or height, except in exceptional circumstances.
3. On the other hand, individualism means the recognition of the agency of a person, that a person's decisions or will start with him or herself. It constitutes a "stopping point" for inquiry about causes. If we believe in individualism, we are implicitly saying that the will of the human biological organism and mind are the ultimate cause of events.
4. Individualism for some has to do with uniqueness. In Monty Python's Life of Brian, there is a scene in which Brian stands on a balcony exhorting a crowd who has followed him home to all "think for themselves". "You're all individuals", he says. And they all chant in unison, "We're all individuals", contradicting what they say by how they are saying it. And softly in the back, you hear one person pipe up, "I'm not", also contradicting what he says by how he is saying it.
So, individualism means not being like others. Of course, this is a difficult standard to reach in an ever more populated world. So, some people claim individualism based on the fact that their thoughts and actions are theirs and no one else's. This doesn't really solve the problem, but just pushes it back one step.
5. Individualism for some means having a unique soul or mind. This is a combination of 2 and 3, along with a metaphysical twist which says that there is a thing, a soul or mind, which really exists and that is the "true" me. Essentially, this means that there is a split between the private world (the world of the true me) and the public world (the world that it "out there", the result of the actions of all the "true me's" in the world).
6. Another version of individualism is self-reliance. The "rugged individual" is the person who does not depend on anything but his or her own resources. One might ask exactly how far this goes (does it mean withdrawing from society, for example?), but despite this vagueness, it is a common sense of the term.
7. Following upon this, individualism can refer to freedom, or lack of external influence over one's actions.
8. One version of individualism is self-interest. One might think that I should always strive for my own best interests (or more radically, that in fact I always do strive for my own interests, whether I recognize it or not). All communities are, therefore, assemblies of people acting in their own interests, and if a community does not seem to be acting in my interests, I can voluntarily withdraw from it. This is a form of atomism, in which society is imagined as a collection of atoms (human beings expressing their preferences) which sometimes come together, and sometimes do not. Ultimately there is no logic above the preferences of those atoms.
A good example of this: Orlando driving. Self-interested individualism, in which traffic laws and customs (like lining up for a right-hand turn rather than racing to the front of the line and then wedging one's way in) can be ignored if they do not further one's own goals.
A slightly more refined version of this is Ayn Rand's philosophy of rational egoism. She believes that we must always recognize that we act in our own interests.
9. There are other senses of individualism. Ethical individualism, for example, holds that the beneficiary of ethical action must be the individual, not some other entity. Political/judicial individualism holds that people may in fact be different, but they must be treated as if they are all identical (see #2, above).
10. Existentialism tends to be a kind of individualism as well. Existentialism holds that we do not come into this world with a definition of who we are, but rather we define ourselves through our actions. There is nothing inherent in individualism that suggests equality between individuals. Friedrich Nietzsche, for instance, believed that some individuals were "higher" than others, and that the lesser individuals created a society that dragged the stronger ones back. John Stuart Mill, on the other hand, did not believe this at all, and argued that blacks or women are not inherently inferior (as others in his time believed), since we are all defined by how we act, not by anything internal to us.
11. Finally, we might think about what individualism is against. Each of the definitions given suggests an opposite. Usually, though, in contemporary political talk, individualism is contrasted to communalism, or the belief that there is an entity apart from individuals (e.g., society) that has agency, or that should be the beneficiary of our actions.
It is worth noting that individualism is an "ism", a belief, a way of organizing the world. To this extent it is not obviously true, but needs to be defended. And, in keeping with the idea that the multiple stories we tell actually help form modern society, it is worth asking how other stories about who we are clash with the story about the individual, and ultimately bring about the world we live in.