The key to understanding the idea that there may be no such thing as human “races,” but that there is racism, is the concept of social constructivism.
Human beings do not access reality directly, but through mental concepts about what we perceive. These concepts are usually transmitted through various means of communication. There is a difference between what is out there (objective reality), what we perceive about it (phenomenology, from the Greek word for appearance), how we sort out this kind of knowledge (epistemology, from the Greek word for knowledge), how we contextualize it into a domain of knowledge (theology, from the Greek word for gods; or philosophy, from the Greek word for the love of wisdom; or science, from the Latin word for knowledge). These knowledge domains sometimes compete with each other, but more often than not, they can cooperate.
Science, humanities and religion have worked together more frequently than we would assume. All have one approach in common: In order to deal with the difficulty that we cannot access reality directly but only through mediation via our senses or our instruments, we create a model of reality in our minds. These models may or may not directly map onto reality directly, but they can be helpful in certain situations. One of these approaches to model reality is the creation of mental constructs, which may be shared throughout a specific culture or society. These concepts are artificial, but ideally not arbitrary. Language is such a model. Linguistics tells us that there is a relation between an object and the descriptor, but that this relation is entirely arbitrary. Some words (like “bump”) may mimic a phenomenon, but not all do, and how a specific language chooses to represent a specific phenomenon varies from language to language.
Whether language is the key to human thinking can be debated. One school of thought certainly believes that, and sees in the way we use language a key to what we are thinking; and sees our thinking eventually tied to our doing. That may sound plausible, but skepticism is also possible, especially when similar actions can occur across cultures despite the use of completely different languages. Furthermore, there is a hot debate over how much our actions are influenced by our thinking, or in how far our thinking just serves a justification of our actions. (Personally, I tend to be skeptical with regards to how language influences thinking, and how thinking actually influences action. Human beings may be much more instinctual and unreflective about their actions than we would care to admit. But my feelings here are beside the point.)
How we utilize language is not the problem, but that our mental assumptions about and models and constructions of the world around us – constructed culturally and socially – can indeed exist quite independently from objective reality. Human beings, like probably most living things, are pattern-seeking creatures, so that we overcorrect for danger (mistaking a bush for a tiger saves a life) rather than to underestimate it to our detriment (mistaking a tiger for a bush may kill you).
We see faces and shapes in the clouds, in rocks, in the sea, in the stars. There is no science to Zodiac signs, and still many otherwise intelligent people claim to believe in them as the basis for astrology. The typical “white” person has a skin color ranging from extreme pale to rosy to red to brown, and still we use the term “white.” Hardly any black person is literally black, and we still use that term. Biology says that “race” does not exist in humans, and yet we think it to be meaningful enough to discriminate against people. This is all about ideology, about constructions of power.
Biological men and biological women are rather similar in most aspects, especially very early and very late in life (when hormonal differences are less important), and yet we proclaim to see essential differences. Boys used to wear pink, and still we think of pink now as a feminine color. Whether you get sick or not may not be up to you, and we still judge people for it. Unless you die young, we all age, and yet discriminate against the old as somehow different people.
We differentiate between people and think in differences because that may well somehow be how we function as humans; but what kind of categories are important may well be arbitrary. Mountains exist, but whether they are considered holy or should be hollowed out for mining depends on what value we attribute to them. What makes big islands like New Zealand and Greenland an island but Australia and Antarctica a continent, and what makes Europe-Asia-Africa three, and America sometimes one, two or three continents, is anyone’s guess. We know that no serious scientist or educated person after the 5th Century BC believed that the Earth was flat, but the lie that people before the 19th century believed the Earth was flat perpetuates our imaginations still because it makes us feel good as somehow advanced, and that’s why Washington Irving invented it.
All of this shows that there are some mental models, or constructions, that we believe in as real, in spite of all the evidence to the contrary. If they guide society, they can be called social constructions. The reality and the phenomena behind them may well be real, but we over-exaggerate their meaning and, in effect, create a difference that holds a meaning to us that – in reality – it should not.