What do we mean when we say that something is real? What ought we to mean? I offer the following definition, in the interest of making an operational notion of reality: an entity is real to the extent that there are operationally coherent activities that can be performed by relying significantly on its existence and its properties. Reality (in the sense of real-ness) conceived in this way is a matter of degrees, and it is domain-dependent. Real entities (or, realities) are mind-framed, and crafting better realities is an achievement of conceptual engineering. When we take ontology in the context of practices, ontological pluralism no longer appears absurd: there are different sets of realities operative in different systems of practice. I illustrate these points with a range of examples drawn from the history of the physical sciences. The traditional picture of physical objects constituted as mereological sums of immutable building-blocks is unwarranted and creates undue hindrance to practice-based ontology.

I propose to understand knowledge in the context of action, refocusing epistemology in order to make it more suitable for engaging with actual practices in science and other realms of life. What I call ‘active knowledge’ is a matter of ability; active knowledge is a prerequisite of propositional knowledge, and propositional knowledge contributes to it. I offer an analysis of active knowledge as operative in ‘epistemic activities’ and ‘systems of practice’ carried out by purposive epistemic agents. The quality of active knowledge consists in ‘operational coherence’, which is a matter of doing what makes sense to do in order to achieve our aims. Inspired by Dewey, I see inquiry as an effort to increase operational coherence, a process in which no aspects of our activities are immune from revision. Generally my thinking is inspired by pragmatism, which I take as a relentless kind of empiricism, which insists that the full lived experience of epistemic agents is the only source of any learning, including learning in logic, methodology and metaphysics.

There are many different things we mean when we say that something is true. What is the relevant sense of truth operative in actual practices, and in the actual judgements that we make concerning the epistemic quality of propositions? It is helpful to start with a distinction between secondary truth (which is grounded in the truth of other propositions), and primary truth (which is not). Concerning the meaning of primary truth in empirical domains, I propose the following definition: a proposition is true to the extent that there are operationally coherent activities that can be performed by relying on it. This ‘truth-by-operational-coherence’ is not a matter of binary yes-or-no, but a quality with many dimensions. It also provides secure underpinnings for epistemic pluralism: mutually incommensurable systems of practice can each contain a set of propositions that are true-by-operational-coherence. Through this notion of truth we can also rehabilitate James’s pragmatist theory of truth.