Science
Socialism and science go hand in hand. Both philosophies view the universe as consisting of real, measurable matter and energy, and nothing else - no mystical spirits or magical forces, no afterlife. The focus is on our world and our lives here and now.
The scientific mindset involves an open-minded attitude but also a heavy dose of skepticism. Beliefs should not be rejected outright, even if they sound crazy - once in a while, a crazy belief turns out to be true. For example, natural selection and evolution demonstrate that species change over unfathomably long eons; you have ancestors who were fish, if you go far enough back in time. Or even for people who readily accept evolution, it may be surprising to hear that a human and a chimp are more genetically related than a mouse and a rat. Or that a T-rex is more closely related to any modern bird than it is to any other dinosaur.
On the other hand, no belief should be accepted at face value without evidence - often things that seem obvious to everyone turn out to be false. It seemed obvious for most of human history, for example, that the Earth is flat, or that the sun is totally different from any other star. The thing is, if everything were crystal clear at first glance, there would be no need for science. Reality can appear misleading, though, and science peels back the illusions in search of truth. The scientific method is the best method - and the only method - that we have for figuring out how the universe really works. Even if we can never gain absolute certainty, we can at least get closer and closer approximations of the truth.
So what's this have to do with socialism? As an economic theory, socialism peels back its own layers of illusions to penetrate the truth about human nature and economics. Capitalism and free trade would have you believe that all people are free agents, voluntarily selling their labor for money in a fair trade, mutually beneficial for everyone involved. Marx demonstrated long ago that such a transaction is not voluntary; it is economic slavery, not much better than legal slavery.
If you don't like your employer, sure, you can quit. But where are you going to go? You need to work somewhere to have money just to survive. And other employers aren't going to pay you much more than before. In fact, they'll pay you as little as possible, and you're competing with your fellow workers for jobs and wages.
This isn't the place for a lengthy essay on Marxism and the alienation of labor; the point is merely to show that, much like religion, capitalism and its apologists will try to pull the wool over your eyes with assumptions and arguments that do not hold water. The purpose of science is to discover truth and understand the universe in which we live - whether we like what we find or not. The purpose of socialism is to restructure society to be more in harmony with human nature and end oppression and inequality. A quote from Trotsky sums up the relationship more eloquently than I can hope to do:
"From the Marxist point of view ... the end is justified if it leads to increasing the power of man over nature and to the abolition of the power of man over man."
That is to say, genuine socialism consists of, and thrives on, two things: (1) science and (2) democracy.
