Human knowledge has been accumulating for hundreds of millennia and has radically transformed our physical environment. And yet the growth of knowledge is itself a purely abstract phenomenon, which consists entirely of ideas interacting with other ideas in a mysterious process that we know involves some form of evolution. A thinking mind is a dynamic environment in which the units of evolutionary selection and the selecting entities are all ideas. Ideas mutate and then differentially “survive” based on the selection pressures that other ideas in that same environment impose. How well any particular mind accumulates objective knowledge depends on how well the ideas in that mind impose upon each other the rational selection pressure of truth-seeking criticism. That is, if mutating ideas in a mind are differentially selected as a function of how well they survive truth-seeking criticism, then they can evolve to become more true. The need for mutation arises when ideas in the mind conflict, because such a conflict can be resolved only by creatively mutating the ideas and then applying truth-seeking criticism to the variants. If the criticism is truth-seeking then it will at each stage select the variants that most successfully resolve the existing conflict. Continuous repetition of this process constitutes a form of evolution in which all of the ideas involved—the conflicting ideas, the mutating ideas, and the criticizing ideas—can improve, in the sense that their objective contents and structure can become truer representations of objective reality. The philosopher Karl Popper called this process problem solving, and characterized “science” as a special case of this process, in which percepts give rise to new conflicts and criticisms, enabling us to accumulate knowledge about the physical world.