From Hume to Kant (via Descartes and Leibniz)

Hume’s Enquiry

Hume’s Enquiry Concerning Human Understanding provides a view of the value and the limits of metaphysical investigations that differs significantly from the views we see in Descartes’ Meditations, Spinonza’s Ethics, and Leibniz’s Monadology.

Alth Descartes, Spinoza, and Leibniz disagree with one another in important ways concerning particular questions in ontology, they share substantive commitments concerning the intellectual powers of human beings and the relative weight given to intellectual representations in relation to those that are generated through sense experience. In addition to claiming that the mind is better known than the body, Descartes claims that the mind or the intellect is active in ways that are necessary for us to make the kinds of judgments concerning bodies that we do–e.g., when we judge (correctly or not) that there is a piece of wax that undergoes changes in its sensible qualities when it is melted, instead of making the judgment that the hard, cold, sweet-smelling thing¹ was annihilated and some warm, liquid thing² was created in its place.

Intellectual Ideas and Knowledge of Things-in-Themselves 

For Descartes (Spinoza and Leibniz) the fact that we have ideas, like the idea of substance, that cannot be traced to some origin in the senses provides evidence that the intellect or mind plays an active role in making judgments about the objects of our sense experience. The independence of the source of these ideas also frees us to use them in thinking about the difference between things as they are perceived by us through our senses and those same things as they exist in themselves; i.e., independently of the particular ways that they happen to appear to beings like us. If there is some existing thing, or substance, that remains through all the changes in sensible qualities that we perceive, there must be some way of characterizing what that thing is that doesn’t refer to any of these changing sensible qualities. gives us a characterization of what the wax really is as it exists in itself and is conceived through itself by the intellect (i.e., its primary qualities) and distinguishes it from our characterization of it as it exists in relation to us and is concerived through our sensory perceptions (i.e., its secondary qualities).  The former makes use of the idea of an extended thing (res extensa). Like all bodies, the wax is extended in three dimensions. It is extended before it is melted, and it is still extended after it is melted, no matter how much the particular quantities of its extension (e,g, its shape and its size) have changed. To conceive of the wax as it exists in itself is to conceive it by way of the intellect’s idea of extension (which is clear and distinct). When we perceive the wax as being hot or cold, soft or hard, sweet-smelling or having no smell, we represent the wax, not as it really is in itself, but as it happens to appear to our senses (somewhat obscurely and confusedly).

Leibniz provides an alternate account of what the wax really is, or what bodies in general really are. Spatial extension is not a fundamental attribute of substance, according to Leibniz. When we think of some thing as being extended, we aren’t thinking about it as it exists in itself by way of a purely intellectual idea. Instead, we are representing it abstractly (i.e., without any of its particular sensible qualities, purely as a geometrical object) through the imagination. The imagination allows us to represent the outward forms of things, but not their inner being. The property of being extended is itself dependent on, or grounded in, some more fundamental attribute of the substance, or substances, that both characterizes these things as they are in-themselves and grounds the extension of bodies. For Leibniz, what is fundamental to a substance is its primitive forces or powers.

This brings Leibniz’s view of the substances in which the extension of bodies is grounded closer to the view that Descartes’ holds of the self, the mind, or thinking substance (res cogitans); i.e., substances are what they are in themselves and can be conceived through themselves in terms of their fundamental active powers. We are aware of our own powers of representation and desire, or perception and appetition, through the cogito; that is, through our conscious (or apperceptive) exercise of these powers in thinking and willing (I think). This provides the basis for thinking the substances responsible for the extension of bodies as also being active through powers of perception and appetition that are analogous to our intellect and our will. There is no real difference in kind, for Leibniz, between the simple substances that make up bodies and the simple substances that we are as human intellects. The difference is simply one of degree with respect to the clarity and distinctness of the representative states that are grounded in these characteristic active forces or powers.

Hume’s Approach to ‘Intellectual’ Ideas

Hume treats ideas that cannot be traced to some origin in the senses in a very different fashion. He famously claims that we have no impression of substance, but he does not think this means the idea stems from some pure power of intellect. It is not evidence that the mind plays an active role in making judgments about the objects of our sense experience. It does not allow us to think about these objects as they are in themselves. It means, instead, that the idea is suspect. Unless we can trace it back to some impression (whether a sensible impression or an impression made on the mind through force of habit or custom), we should remain suspicious of its use in philosophy. In the Enquiry, Hume doesn’t focus on the idea of substance, but he does focus on the idea of causality and on the idea of power or force, which, as we see in Leibniz, are close correlates of the idea of substance.

The impression from which these ideas stem is not an impression of the sensible qualities of some body or of the observable relations between bodies; we don’t see, hear, touch, taste, or smell the force or power through which a cause (e.g., the motion of body¹) produces an effect (e.g. the motion of body²). Hume argues that the same holds for so-called mental causation. We are aware of a desire to raise our hand and then we are aware of our hand moving, but we don’t perceive the mental power by which we bring this about. We are aware of a desire to think about a friend, and then we perceive ideas of the friend, but we don’t perceive the power through which these ideas are generated.

The impressions from which these ideas are produced are, instead, the feelings that arise in the mind when it perceives one thing and infers the existence of some distinct thing; that is, when it perceives the cause and infers the existence of the effect or perceives the effect and infers the existence of the cause. To put it another way, for Hume, we come to have these ideas, not through a self-conscious awareness that accompanies the activities of the mind, but through a sense of our own passivity with respect to the connections we are forced to make between our perceptions and those distinct objects that we do not perceive to which the perceptions are related. We become habituated in our perceptions through experience to such a degree that we cannot but follow the patterns that these experiences have ingrained in us. We gain the capacity to ‘reason’ about real existence and matters of fact in this way. We can think about and draw conclusions concerning real existing things as causes in relation to perceived events or states of affairs, because we have been trained by experience concerning what kinds of things there are and what kinds of causes and effects these things have. We cannot think about or know these existing things as they exist in themselves, independently of all relation to our senses and to the objects of our experience. We do not, for Hume, have an active intellectual power that enables us to conceive and to cognize things-in-themselves through themselves.

Kant’s Reaction to Hume

Part I: General Considerations

  • Hume’s Skepticism concerning the power of intellect/Naturalism concerning knowledge and action
  • Hume’s reorientation of metaphysics: from rational theism and natural theology to naturalistic description of human understanding
  • Kant’s understanding of Hume’s challenge for metaphysical thinking
  • Partial response to the challenge already available in geometry and physics
  • Model of the mind required to understand the kind of cognition possible in geometry and physics provides clue to how metaphysical cognition is possible

Part II: Kant’s Model of the Mind

  • General Class = Representations (Instead of Cartesian Ideas or Humean Perceptions)
  • Distinguishing Characteristics = Formal Features and Functions (instead of Degrees of Clarity and Distinctness or of Force and Vicacity)
  • Intuitions, Concepts, Ideas
  • Sensibility, Understanding, Reason
  • Actions in Judgments (thinking, distinguishing, determining)
  • Judgments and Empirical Knowledge

Part III: Synthetic Judgments (a priori and a posteriori)

  • Analytic and Synthetic Judgments
  • Synthetic Judgments a posteriori and empirical intuitions
  • Synthetic Judgments and construction of ideal objects by way of a priori intuitions (space and time)

Kant’s Critique of Reason

Part IV: The Theoretical Restriction of Reason and Traditional Metaphysics

  • Synthetic a priori judgments and the conditions for the possibility of experience
  • Applicability of a priori principles to empirically given objects
  • Restriction of determinate application of principles to real or possible empirical objects (phenomena)
  • Demands of reason and unconditioned conditions
  • Contributions of sensibility, understanding, and reason to the possibility of synthetic a priori cognition of nature
  • Reason and Traditional Metaphysics

Part V: The Practical Extension of Reason and Traditional Metaphysics

  • Limits to theoretical reason’s ability to determine answers to questions about the objects of its Ideas
  • Source of reason’s interest in moving beyond theoretical considerations
  • Immortality of the Soul as Practical Postulate
  • Existence of God as Practical Postulate
  • Metaphysics of Nature and Metaphysics of Morals

Forward: Part I: General Considerations