Intuitive knowledge is of greatest certainty by immediate perception of the mind without others intervention. In intuitive knowledge, the mind understand or know something immediately without needing to think about it, learn it or discover it by using reason. The mind identifies the truth without having to prove or examine ideas. By direct reasoning, it perceives that human is different from a dog, a circle is not a triangle, three are more than two. The mind identifies the agreement or disagreement of two ideas by their own immediately, exclusive of others’ interference. Intuitive knowledge is the clearest and of most certainty, with no double nor hesitation. It is irresistible and immediately perceived by the mind. The certainty of intuition is so great that one cannot conceive. As a result, a greater certainty is not needed. The existence of ourselves is also an intuitive knowledge. Our consciousness implies our existence. It is not capable or need any proof. This is because nothing can be more certain than our own existence.
Demonstrative knowledge is the next degree of knowledge that something is proved and explained. The mind identifies the agreement or disagreement of ideas, but unlike intuitive knowledge, this process is not immediate. Whenever the mind perceives the agreement or disagreement of ideas, there is knowledge. Yet, the mind cannot always perceive immediately. In this case, intervention of other ideas is needed. These intervening ideas are the proofs that demonstration depends on. When these proofs are clearly perceived and show the agreement of two ideas, demonstration occurs. To prove the sum of interior angles of a triangle is 180 degree, we first calculate each angle of the triangle and add them together. We then find out the sum of the angles are always equal to 180 degree. We also know that the degree of a right angle is 90 degree, so two right angles are equal to 180 degree. Eventually, we can conclude that the sum of interior angles of a triangle is equal to two right angles. With proofs and reasons, demonstrative knowledge is certain, yet it is not as ready and easy as intuitive knowledge. More than one transitory view is needed to find the agreement of ideas. Although demonstrative knowledge is certain, before the demonstration there was a doubt. Certainty is reached by going through steps of intuition with progression of degrees.
Sensation can prove the existence of external objects even without us. Locke argued that all our knowledge is founded by experience, thus we could not have knowledge without sensation. Locke believed that sensation is a source of ideas, ‘…our Senses, conversant about particular sensible objects, do convey into the mind several distinct perceptions of things…’. Ideas are conveyed into our mind through our senses, such as our vision and our smell. We can see whether an object is blue or yellow, or heat or cold. These are the ideas of sensible qualities. The ideas that we perceive from our senses are the “raw materials” for our minds to generate more ideas and to think of. Locke was certain that sensation prove the existence of external objects, “perceives the truth as the eye doth light, only by being directed towards it”. We simply cannot control what we see or sense, once we open our own eyes. The objects we see or sense now must exist before our eyes at this very moment, unlike memory or imagination, which the objects do not necessarily exist. By sensation, it is certain that the external object existed even without us. Furthermore, the existence of the external object perceived by our senses is consistent with the pleasure and pain received from the same object. Such as when we eat something, we become full, which is a pleasure. But we cannot become full by remembering the food we ate last night, or by imagining we are eating some food. These examples prove that the objects that we perceive the idea of must themselves exist and from which pleasure or pain are brought to us. Therefore, according to Locke, by sensation, the existence of external object is certain by the pleasure and pain received and its differences from memory and imagination.
Lock was certain that human can distinguish reality and dream in response to the dream argument. Descartes rejected empiricism by raising two doubts about reality and dreams. Firstly, he claimed if people were in dreams, all ideas thus were not came from any external objects but produced by the people himself. Secondly, he doubted how people could certainly know they were actually experiencing certain events or they are only dreaming the experience. Locke responded to the first argument by suggesting the importance of reality to the person in dream. If a person lives in the dream eternally without waking up, reality is of no importance to him anymore. Reasoning and demonstrations are not able to help the person to gain knowledge, whereas knowledge and reality are meaningless to the one who stays in dream forever. Locke was also certain that human could distinguish real experience and being in dream. Human have the senses to perceive external objects and from them we receive pleasure and pain. For example, the pain I receive from hitting by Dr. Cheng in dream is different from actually hitting by him. The pleasure and pain are as great as happiness and misery that for certain is different from being in dream. We review the experience in dreams in real life and for sure it is unclear and gives us no pain.
To conclude, sensation and reflection gives us ideas, and by knowing the relation of ideas, we have knowledge. Sensation is the foundation of all other knowledge and by reasoning, we can have intuitive and demonstrative knowledge. The consistence of experience and direct perception of external object by sensation proved the existent of that object. By acknowledging the difference of pain and pleasure received from the real world and in dream, we can definitely realize we are in dream of not.
