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