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The senses as our receptors to the outside world were however split into two categories depending on their quality of perception. These are described as the primary and the secondary qualities of the senses. Locke made a basic distinction between the two, arguing that firstly, Primary qualities are entirely inseparable from body. They are known to be primary because the senses consistently find them to be the same, in other words they are perceptions that cannot be altered by factors such as heat or light, which might, for example, affect the color which the object or stimulus could be perceived. Therefore solidity, shape, and size are all primary qualities. The Secondary qualities, in Locke's terms, are qualities of perception that could be perceived differently and are dependable on surrounding factors as well as the individual themselves. Consequently effects such as color, odor, sound, warmth, and smell are all secondary qualities. These qualities can be seen in instances when an individual may perceive differently at different times. For example water, which may appear warm to someone who is cold, might be perceived as cold to someone who is hot. Nonetheless it is clear that Locke saw the senses as the basic receptors to learning from the experiences. .
Locke also defined the two sources of knowledge as sensation and reflection. Locke discussed the modes of the simple ideas of reflection such as remembering, reasoning and judging. He claims that during the process of reflection, the mind observes its own action. .
As mentioned before Locke as an empiricist contended that there are no innate principles stamped upon the mind of human being, because principles such as "Whatever is, is" and "It is impossible for the same thing to be and not to be" are not innately known to people, but are a result of knowledge through learning. On this basis Locke assumed that for something to be in the mind it has to be perceived or be readily recalled in order to be knowledge.