In addition to the vital awareness of God and morality that guides proper action, practical human knowledge requires some familiarity with the physical world whose features so clearly limit our capacity for happiness. Thus, Locke devoted careful attention to our use of the faculty of sensory perception, the passive ability to receive ideas from the external world by means of our sensory organs. The simple ideas acquired by attending to these mechanical operations are the first components of all human thought. [Essay II ix 1-6] Although sensitive knowledge is strictly limited in its scope and reliability, Locke held that the testimony of sensory experience, lying as it does entirely outside our voluntary control, provides all the evidence we can haveand all the evidence we needof the existence, nature, and operation of physical objects. [Essay IV xi 2-6]
Since the simple idea of space is derived from both visual and tactile perception, it is conceived as the three-dimensional separation of distinct bodies. Its simple modes include both units of spatial measurement and the geometrical figures. [Essay II xiii 2-6] Because space is conceived as continuous extension with inseparable and immoveable parts, Locke emphatically denied the Cartesian identification of space with body. [Essay II xiii 13-17] This has significant consequences, including the possibility of space without any body in itthe vacuum so decried by Cartesian natural philosophy. On Locke's view, the argument is clear: since the ideas of space and body are distinct, the possibility of one existing without the other is evident, unless our opinions are driven by theoretical prejudice. [Essay II xiii 21-26] The further idea of place is a simple mode of space as specified by reference to fixed points or objects within some frame of reference, chosen for its convenience in serving human purposes. [Essay II xiii 7-10]
Time yields to a similar pattern of analysis. Reflection on the succession of ideas in the mind, especially during our experience of the motion of external objects, gives rise to the simple idea of duration, from which in turn we derive all of the simple modes of time and temporal measurments. [Essay II xiv 1-8] The idea of time itself is just that of a determinate length of this duration, measured across non-contemporaneous intervals by reference to the presumed regularity of periodic natural movements, such as the rotation and orbit of the earth in relation to the sun. [Essay II xiv 17-23] Once established upon the basis of our experiences, these measurments of time can be applied beyond the limits of that experience, or even beyond the finite boundaries of the original motions themselves. Once we know how much time "a year" is, by observation of the seasonal changes produced during a single orbit of the earth, we can without difficulty consider the extent of time that occurred before the sun or earth existed. [Essay II xiv 24-30]
There is, then, a systematic analogy between spatial and temporal ideas for Locke: Both are conceived in limited experiences, yet can be applied to infinite expansion or duration. Space and duration are continuous and undifferentiated, yet both permit the designation of place and time in relation to fixed points of reference. Finally, both space and time are infinitely divisible, even though their parts are inseparable. Together, they provide a framework for organizing our experience, within three-dimensional space and linear time. [Essay II xv 1-12]
Both space and time are subject to quantification. Beginning with the simple idea of unity, each of us repeats and compounds it mentally in order to conceive the simple modes of number, each of which is so clear and distinct as to ground demonstrable knowledge of mathematics. [Essay II xvi 1-8] What is more, Locke held that this process of compounding is, in principle, unlimited, and our awareness that the process could be repeated indefinitely provides us with a clear idea of infinity. [Essay II xvii 1-5] Although we naturally and correctly apply this idea to our conceptions of space and time, Locke noted that we have no positive notion from experience of either infinite duration or infinite extension. Knowing that there is no end of the counting process is not the same as having an independent, positive idea of infinity. [Essay II xvii 9-15]
Within the spatio-temporal framework, what we experience are physical objects, or bodies. Unlike the Cartesians, Locke distinguished sharply between the ideas of space and body: both involve extension, but bodies have the additional feature of solidity. Derived from our tactile experience of the physical world, the simple idea of solidity is that of mutual impenetrabilitywhere space is filled by one body, another cannot enter. This idea is different both from the simple idea of extension and from that of hardness, but solidity alone is an inseparable essential feature of all bodies. [Essay II iv 1-6] The experience of changes in the relative place of distinct bodies with respect to each other through time gives rise to the idea of motion, and this is another of our primary ideas of bodies. In practice, we commonly distinguish different bodies by moving them apart from each other spatially. [Essay II xiii 7-11] But Locke emphasized that motion is only an activity of bodies, not truly a part of their essence. [Essay II i 10]
Thus, the most basic features of body are extension, solidity, and mobility. When determinate qualities of these determinables (along with the ideas of the powers of causal interaction) are added to the idea of substance in general, we form the complex idea of a physical object. [Essay II xxiii 3-10] Although Locke believed our knowledge of material substances to be strictly limited by the bounds of our sensory capacities, he also supposed that the experiential awareness of bodies that we are capable of is adequate for our practical needs for living successfully in a material world. [Essay II xxiii 11-16]
Although we are incapable of demonstrative knowledge of the substantial natures and causal powers that operate in the natural world, on Locke's view, we can achieve probable knowledge based upon our sensory observations. The corpuscularian hypothesis offers our most coherent account of the occurrence of observable sensible qualities. [Essay IV iii 16] The insensible, minute particles of material substance are the real essences whose causal interactions, in ways unknown to us, produce all of the observable qualities and powers that come to be included in our abstract ideas of them, their nominal essences. [Essay III iii 17-18] Analogical reasoning from familiar macroscopic events enables us at least to imagine how unobservably small corpuscles might interact in ways that produce the sensible qualities and powers of bodies that are visible to us. [Essay IV xvi 12]
Thus, our knowledge of the natural world and its operations is reliable, adequate, and sufficient for our practical needs, yet it is strictly limited. Efforts to achieve a more systematic degree of certainty in this realm are not only bound to fail but also tend to require a blind assent to abstract principles, which can only confirm us in error rather than lead us to truth. [Essay IV xii 4-5] Here, as elsewhere in the development of the human understanding, Locke believed it vital to recognize and to respect our limitations.
Our ignorance of nature extends to the operations as well as to the natures of bodies. The constancy of our observations of simple interactions among physical objects, for example, leads us to suppose that there must be some genuine degree of causal regularity in nature itself, this awareness can never reach to more than the level of probable assurance, since the causes themselves remain unknown to us. [Essay IV xvi 6] The ideas of cause and effect derive from our observation that changes appear to occur as the result of other changes. What we identify as capable of producing such a change, we call the cause, and the change it produces, the effect. Within this general conceptual frame, Locke distinguished between the creation of something entirely new, the generation of a natural being from its own internal development, the making of an artificial being, and the alteration of things by substituting one quality from another. [Essay II xxvi 1-2] In none of these instances, however, are we fully aware of the nature of the causal process itself.
Such assurance of causal regularity as we can achieve derives solely from the observed recurrence of patterns in our sensory experience, and when the experiences are confirmed with universality, our confidence inevitably grows. Thusly encouraged, we use analogical reasoning to generate causal hypothesis about connections among things and events whose operations lie wholly beyond our experience.
[Essay IV xvi 6-7]
But such suppositions can never be certain. Our ignorance of the primary qualities of bodies and of the means by which they produce observable secondary qualities make it impossible for us to demonstrate the coexistence of qualities in a common subject, even when we have observed it frequently enough to incorporate putative causal powers within our nominal essences of substances of a particular sort.
[Essay IV vi 6-9]
So long as our knowledge of bodies is dervied from the observable qualities of bodies, in ignorance of their internal features and operations, we can have no certain universal knowledge of the material world. Without doubting the genuine causal efficacy of particular substances, Locke believed it impossible for us to know of it. Observational regularity is the best we can do, and it must be adequate for our needs, even though we remain forever incapable of comprehending the true structure of reality.
[Essay IV vi 10-11]