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Human understanding requires examining its own powers before tackling ultimate mysteries. A mariner needs a sounding line to know where shallow water threatens, even if he cannot fathom the ocean's depths. Determining the limits of what the mind can know prevents unproductive skepticism and fruitless speculation.
Universal consent does not prove innate ideas, because no principles exist to which all mankind assents. Infants and idiots have no notion that whatever is, is, or that a thing cannot be and not be at the same time. If these logical axioms were stamped upon the soul at birth, every conscious mind would naturally register them.
Moral rules require reason and justification, which proves they are not imprinted on the mind at birth. Different human societies uphold entirely contradictory practical virtues and customs without universal remorse. No moral rule commands assent in the same manner as an immediate sensory perception.
Concepts like God, identity, or substance are missing in newborn infants and absent in entire cultures. An idea cannot exist in the mind without that mind being conscious of it. All raw materials of thought originate only after experience begins to imprint them.
The mind is initially white paper, void of all characters, until experience writes upon it. Sensation provides ideas of external objects, while reflection provides ideas of the mind's own internal operations. Every complex thought ultimately traces back to these two distinct fountains of knowledge.
Simple ideas enter the mind unmixed and clear, as when coldness and hardness arrive together from a piece of ice. The mind can neither fabricate a new simple idea nor destroy one already received. Human invention combines or divides these elementary units, but cannot create their basic sensory content.
Certain simple ideas enter the mind through one specific sensory organ alone. Colors come only by sight, sounds by hearing, and heat by touch. A person born blind cannot form the idea of scarlet, as the corresponding entrance remains closed.
Solidity arises from touch, as when two marble globes press against each other and prevent mutual penetration. This property differs from hardness, which measures resistance to rearrangement of parts. Solid space remains filled, keeping all other bodies out regardless of applied pressure.
Shape, size, motion, and rest enter the mind through more than one sensory pathway. Sight and touch both convey the figure of a globe or the movement of a rolling stone. These shared perceptions build the spatial architecture of the external world.
Reflection occurs when the mind turns its view inward upon its own actions. Thinking and willing constitute the primary actions observed through this internal observation. Observing oneself remembering, doubting, or choosing yields distinct simple ideas not derived from external objects.
Pleasure, pain, power, existence, and unity accompany almost all sensory inputs and mental reflections. A flame produces both the idea of light through sight and the idea of pain through contact. Unity attaches to every single object perceived or thought of by the mind.
Primary qualities like shape, extension, and solidity belong inherently to objects whether perceived or not. Secondary qualities like color, taste, and warmth are merely powers in the object to produce sensations in us. A snowball causes coldness and whiteness in the viewer, but the coldness itself is not inside the snow.
Perception is the first step toward knowledge and the gateway for all mental content. Ideas in the mind require actual conscious awareness; without perception, sensory impressions remain unformed. Alterations by judgment often modify raw visual input, as when a shaded circle is perceived as a three-dimensional globe.
Retention preserves simple ideas through contemplation and memory. Contemplation keeps an idea present in active thought, while memory acts as a repository for dormant ideas. Without periodic renewal, ideas fade like inscriptions carved into marble exposed to weather.
Discerning distinguishes one idea from another, preventing confusion in thinking. Comparison, composition, and abstraction allow the mind to group particulars into general concepts. Brutes may retain and compare specific sensations, but abstraction belongs uniquely to human reasoning.
The mind acts passively when receiving simple ideas, but actively when combining them into complex ones. Complex ideas fall into three categories: modes, substances, and relations. An idea like gratitude or a triangle exists only as a combination of simpler elements framed by the intellect.
Space consists of modified repetition of extension, measured by distance and capacity. Length between points creates distance, while enclosed space creates figure and volume. The mind can expand space endlessly in thought, reaching beyond any physical boundary.
Duration is measured by the constant succession of ideas passing through the mind. Sleeping without dreaming leaves no awareness of elapsed time, because no chain of thoughts occurred. Standard measures like hours or days align internal duration with external periodic motions like the sun.
Duration and expansion both extend endlessly, representing time and space respectively. Duration applies to all existing things sequentially, while expansion applies to spatial presence simultaneously. Infinity in either dimension exceeds complete comprehension, though both can be endlessly added to in thought.
Number provides the clearest and most distinct simple mode, applicable to everything in sensation and reflection. Counting adds unit to unit, providing exact precision that spatial measurements cannot match. Without numerical names and symbols, complex reckoning and orderly thought become impossible.
The idea of infinity is an endless growing process, not a completed visual idea. The mind easily adds numbers, spaces, or duration indefinitely without ever reaching a final limit. Confusing the capacity to continuously add with a finished concept of an infinite object leads to philosophical errors.
Simple modes of motion include sliding, walking, and running, each formed by combining variations of movement. Modes of sound include tones, tunes, and spoken words, built from simple auditory sensations. These variations allow complex language and precise descriptions derived from basic inputs.
Thinking manifests in varied modes such as sensation, remembrance, recollection, contemplation, and dreaming. When sensation fades during sleep, the mind constructs dreams from unguided fragments of past experience. Full attention occurs when the mind intently focuses on a single internal idea.
Pleasure and pain are the fundamental motivators of human action and thought. Good is whatever increases pleasure or reduces pain, while evil is whatever produces pain or diminishes pleasure. Emotions like hope, fear, joy, and sorrow are simple modes built upon these twin sensory roots.
John Locke dismantles innate knowledge and traces all human thought to the raw materials of sensation and experience.