John Locke and David Hume, both great empiricist philosophers who radically changed the way people view ideas and how they come about. Although similar in their beliefs, the two have some quite key differences in the way they view empiricism. Locke believed in causality, and used the example of the mental observation of thinking to raise your arm, and then your arm raising, whereas Hume believed that causality is not something that can be known, as a direct experience of cause, cannot be sensed. Locke believed that all knowledge is derived from our senses, which produce impressions on the mind which turn to ideas, whereas Hume's believed that all knowledge is derived from experiences, and that any experience is of one's own perception. Thus not allowing us to be completely certain of anything. Locke also believed that ideas could be created beyond sense perception, through multiple simple sensations, such as the idea of god, whereas Humes believed that without a direct experience no such idea is valid. Although Locke and Hume share a basic belief of empiricism, Hume's view on ideas and the ability to be certain of anything in reality, is restrained to the confines of experiences being perceived differently, or not at all.
Locke believed that despite cause being largely unseen and rather difficult to sense that it was nontheless able to be known. He used the analogy of raising your arm. To raise your arm, you must first think mentally about raising your arm, as you do so you feel your arm raise until you think about your arm stopping. It then stops. Since this sequence of events happens within the mind, you are thus experiencing a cause. Being able to know the connection between cause and effect, and being able to create an idea of it allows for things to be known for certain in reality. For example, knowing the connection between cause and effect allows you to know that whenever water reaches 0 degrees centigrade, it will freeze. Without certainty of cause that the water freezes at 0 degrees centigrade ideas are based off of reoccuring sense experiences that may not happen everytime the water reaches 0 degrees.
Hume rejected lockes theory of experiencing cause. He argued that you do not feel the connection between your mind and arm, and thus don't sense the cause of the muscles contracting to raise your arm. Cause, in Hume's mind, is a synthetic experience used to explain the unobservable things in reality. To help explain he used the billiard ball experiement. Ball A is hit and put into motion towards ball B.When ball A collides with ball B the cause of ball B's movement is not experienced, there is no observable connection between the two. This would mean that there is no way to be certain that everytime Ball A collides with ball B that ball B will move, ball A could just as likely bounce off and begin rolling in a random direction. He believd that there is no way of knowing for certain the outcome of an event without being able to perceive the cause.
John Locke's theory of knowledge stated that all knowledge is derived from the senses, that are converted into impressions, that are then made into ideas, either simple or complex. Simple ideas are ones that involve only one sense, whereas complex ideas consist of multiple simple ideas being combined to create a vivid one. Ideas have two qualities, primary qualities, and secondary qualities. Primary qualities are things that are perceived the same for everyone, and secondary qualities are the individual perceptions of them. For example, two people that look at a cat will sense the same primary qualities of the cat, but one may have had a traumatic experience in the past with a cat and will perceive it differently then the other person. This was Lockes response to critiques that if knowledge was derived from senses everyone would perceive everything the same.
Hume's theory of knowledge was a bit different....