Modern Metaphysics and Epistemology

Kant's synthetic a priori judgments

Kant classifies judgements in two ways. Logically, a judgement is analytic when the predicate is already contained in the concept of the subject, so that denying it yields a contradiction, and synthetic when the predicate adds content that the subject concept does not contain. Epistemologically, a judgement is a priori when it is known independently of experience and carries strict necessity and universality, and a posteriori when its justification rests on experience. Kant's distinctive claim is that mathematics and the principles of pure natural science are synthetic a priori: they extend our knowledge while being necessary and universal.

• analytic judgement — the predicate is contained in the subject concept. For example, 'All bodies are extended' is analytic, because extension is part of what we mean by body.
• synthetic a posteriori judgement — the predicate adds content known only through experience. For example, 'All bodies are heavy' is synthetic, because weight is not part of the concept of body.
• arithmetic — Kant counts 7 + 5 = 12 and 2 + 2 = 4 as synthetic a priori. The concept of 12 is not contained in the concepts of 7, 5 and addition, so reaching it requires an intuition such as counting fingers or marks, which is why the truth is not established by analysis alone.
• geometry — the angle sum of a triangle is 180 degrees, and the straight line as the shortest path between two points, are synthetic a priori truths grasped through the pure intuition of space.
• pure physics — the conservation of the quantity of matter and the equality of action and reaction are necessary principles known a priori that extend the concepts of matter and motion.
• objects of sense — for Kant, objects of experience are never given as bare sensory awareness; they are structured by the forms of intuition, space and time, and by the categories of the understanding.

Statements that report observations or contingent facts about particular objects, plants or chemical elements are a posteriori. Expressions of wish or feeling are not judgements at all, so they cannot be classed as synthetic a priori.

Hume's impressions and ideas

Hume uses perception as the general term for every content of the mind, and divides perceptions into two kinds that differ in force and vivacity. Impressions are the more forceful and lively perceptions, arising in the first instance from sensation, and they include sensations, passions and emotions. In the Treatise, impressions of reflection such as desires and passions are also counted among impressions. Ideas are the faint copies of impressions that we use in thinking and reasoning.

Hume's copy principle states that every simple idea is derived from a corresponding earlier simple impression. Complex ideas can be formed by combining simple ones, for example by imagining a creature that has the head of one animal and the body of another.

• perception — the general name for all the items of the mind.
• impression — a lively perception such as the feeling of warmth from sunlight; the later memory of it is an idea.
• association of ideas — Hume identifies three principles that connect ideas in the mind: resemblance, contiguity in time or place, and cause and effect. Juxtaposition, which is merely the spatial placement of objects, is not a mental principle of association.

Berkeley's rejection of abstract ideas

Berkeley holds that everything we perceive is an idea, and that an idea can be like nothing but another idea. From this it follows that no idea can exist essentially independently of mind. For sensible things, to exist is to be perceived (esse est percipi), and to be unperceived is not to exist in any sense Berkeley accepts.

Berkeley also argues that one cannot conceive of an existing thing apart from its being perceived, since any such conception is itself an idea in the mind. He therefore rejects abstract general ideas. We cannot form an image of a triangle that is neither equilateral, isosceles nor scalene; every idea we form is particular, and it serves as a sign for a class of similar particulars.

Hegel's Phenomenology of Spirit

In the Phenomenology of Spirit (1807), Hegel traces the path of consciousness from sense-certainty through self-consciousness, reason, spirit and religion to Absolute Knowing. Spirit attains full self-knowledge in ethical life (Sittlichkeit), in which individual freedom is recognised and realised in the family, civil society and the state. Religious devotion, natural science and immediate mystical insight are stages of consciousness that Hegel either subordinates to conceptual knowledge or criticises as immediate.

In the Philosophy of History, Hegel holds that world history is the life story of Spirit, whose essential content is freedom. Spirit proceeds from potentiality to actuality through historical struggle, in which it comes to know itself and find itself. History is therefore the logical Idea manifested in the sphere of time and existence, and the internal development of Spirit necessarily realises itself in concrete historical reality.

• Sittlichkeit — ethical life, the realm where free individuals recognise themselves in social institutions.
• potentiality and actuality — Spirit begins as what it is implicitly and becomes what it is explicitly through its own activity.

Logical Positivism epistemology

Logical positivism, associated with the Vienna Circle and with A. J. Ayer and Rudolf Carnap, held that a statement is cognitively meaningful only if it is either analytic or capable of empirical verification. The verification principle of meaning states that a statement has meaning if and only if the fact that it is true would make a verifiable difference to experience.

The movement accepted analytic a priori truths of logic and mathematics, which are known through the meanings of terms, so empirical experience is not claimed to be the sole source of all knowledge. It rejected synthetic a priori knowledge, and it treated metaphysical statements as meaningless. Scientific laws, however, were regarded as genuine universal hypotheses whose content goes beyond the finite observations that support them.

• verifiability criterion — a statement that no possible observation could confirm or refute is cognitively meaningless, which is the basis of Carnap's rejection of metaphysics in his 1932 essay on overcoming metaphysics through logical analysis of language.
• analytic a priori — truths of logic and mathematics that do not depend on empirical facts for their truth.

Locke's primary and secondary qualities

Locke uses quality to mean a power in a body to produce an idea in the mind. Primary qualities are inseparable from body in all its states, and they include solidity, extension, figure, motion or rest, and number. They are perceived by more than one sense, and their ideas resemble the qualities as they are in bodies. Secondary qualities, such as colour, sound, taste, smell and heat, are powers to produce sensations by means of the primary qualities of the body's insensible parts. Their ideas do not resemble anything in the bodies that cause them.

It follows that the claim that all our ideas are exact images of the bodies causing them is incorrect, because it fails for secondary qualities. The same question, whether a quality belongs to the thing itself or depends on its effect on the perceiver, appears in Plato's Phaedo. There Socrates rejects the view that beauty consists in colour, shape or size, and explains that a beautiful thing is beautiful through the presence (parousia) of the Beautiful itself, which exists by itself (auto kath' auto).

Empiricist theories of ideas

Empiricism holds that all ideas originate in experience, either in sensation or in the mind's reflection on its own operations. Locke, the central figure of this tradition, argues in the Essay that the mind is at first like blank paper, and that we know nominal essences, the ideas by which we classify things, but not the real essences of substances. Because the real constitution of corporeal substances is hidden from us, Locke denies that natural philosophy can be true science. He nevertheless holds that the existence of God can be known with certainty by demonstration from the certain intuition of our own existence.

Empiricist and political thinkers also differ on the human condition. Hobbes describes life in the state of nature as solitary, poor, nasty, brutish and short. Locke describes the state of nature as a condition of freedom and equality bounded by the law of nature, while Rousseau begins The Social Contract with the claim that man is born free but everywhere in chains. Marx, by contrast, treats economic structure as the base of social reality, which is a claim of historical materialism rather than of Locke.