Because the concept of knowledge is so central to epistemological theorizing, it is necessary to briefly discuss knowledge before proceeding. Knowledge enjoys a special status among beliefs and mental states. To say that a person knows something directly implies that the person is not wrong, so knowledge implies truth. But knowledge is more than just truth. Knowledge also implies effort—that the person who has knowledge did more than just form a belief; they somehow earned it. Often, in epistemology, this is understood as justification. These features of knowledge are important to keep in mind as we continue. First, we will look at the different ways of knowing. Ways of Knowing The distinction between a priori knowledge and a posteriori knowledge reveals something important about the possible ways a person can gain knowledge. Most knowledge requires experience in the world, although some knowledge without experience is also possible. A priori knowledge is knowledge that can be gained using reason alone. The acquisition of a priori knowledge does not depend on experience. One way to think of a priori knowledge is that it is logically prior to experience, which does not necessarily mean that it is always prior in time to experience. Knowledge that exists before experience (prior in time) is innate knowledge, or knowledge that one is somehow born with. Theorists disagree over whether innate knowledge exists. But many theorists agree that people can come to know things by merely thinking. For example, one can know that 4 × 2 = 8 without needing to search for outside evidence. A posteriori knowledge is knowledge that can only be gained through experience. Because a posteriori knowledge depends on experience, it is empirical. Something is empirical if it is based on and verifiable through observation and experience, so empirical knowledge is knowledge gained from sense perception. If my belief that there’s a bird on the branch outside my window is knowledge, it would be a posteriori knowledge. The difference between a posteriori and a priori knowledge is that the former requires experience and the latter does not. While a priori knowledge does not require experience, this does not mean that it must always be reached using reason alone. A priori knowledge can be learned through experience. Think of mathematical truths. While it is possible to figure out multiplication using thinking alone, many first understand it empirically by memorizing multiplication tables and only later come to understand why the operations work the way they do. Figure 7.2 Some facts that students are asked to memorize in school, such as multiplication tables, fall into the category of a priori knowledge—knowledge gained through reason alone. Knowledge about the shortest route to the nearest restroom, while possibly informed by looking at a map, typically is grounded in a posteriori knowledge—knowledge that can only be gained through experience. (credit: modification of work “Ventura Elementary-12” by US Department of Education/Flickr, CC BY 2.0) Things You Can Know: Types of Knowledge Philosophers classify knowledge not only by source but also by type. Propositional knowledge is knowledge of propositions or statements. A proposition or statement is a declarative sentence with a truth value—that is, a sentence that is either true or false. If one knows a statement, that means that the statement is true. And true statements about the world are usually called facts. Hence, propositional knowledge is best thought of as knowledge of facts. Facts about the world are infinite. It is a fact that the square root of 9 is 3. It is a fact that Earth is round. It is a fact that the author of this chapter is five feet, one inch tall, and it is a fact that Nairobi is the capital of Kenya. Often, philosophers describe propositional knowledge as “knowledge that,” and if you look at the structure of the previous sentences, you can see why. Someone can know that Nairobi is the capital of Kenya, and “Nairobi is the capital of Kenya” is a true proposition. Propositional knowledge can be a priori or a posteriori. Knowledge of our own height is clearly a posteriori because we cannot know this without measuring ourselves. But knowing that 3 is the square root of 9 is a priori, given that it’s possible for a person to reason their way to this belief. Propositional knowledge is the primary focus of traditional epistemology. In the following sections of this chapter, keep in mind that knowledge refers to propositional knowledge. While traditional epistemology focuses on propositional knowledge, other types of knowledge exist. Procedural knowledge is best understood as know-how. Procedural knowledge involves the ability to perform some task successfully. While a person may know that a bicycle stays erect using centrifugal force and forward momentum caused by peddling, and that the forces of friction and air resistance will affect their speed, this does not mean that they know how to ride a bicycle. Having propositional knowledge concerning a task does not guarantee that one has procedural knowledge of that task. Indeed, one could be a physicist who studies the forces involved in keeping a bike upright, and therefore know many facts about bicycles, but still not know how to ride a bike. Figure 7.3 Several forces are at work when a person rides a bicycle. Understanding the physics of cycling does not guarantee that one knows how to ride a bicycle. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license) Knowledge by acquaintance is knowledge gained from direct experience. A person knows something by acquaintance when they are directly aware of that thing. This awareness comes from direct perception using one’s senses. For example, I have knowledge by acquaintance of pain when I am in pain. I am directly aware of the pain, so I cannot be mistaken about the existence of the pain. British philosopher Bertrand Russell (1872–1970) is credited with first articulating a distinction between knowledge by acquaintance and propositional knowledge, which he called knowledge by description (Russell 1910–1911). According to Russell, knowledge by acquaintance is a direct form of knowledge. A person has knowledge by acquaintance when they have direct cognitive awareness of it, which is awareness absent of inference. That knowledge by acquaintance is not the product of inference is very important. Inference is a stepwise process of reasoning that moves from one idea to another. When I feel pain, I am acquainted with that pain without thinking to myself, “I am in pain.” No inference is required on my part for me to know of my pain. I am simply aware of it. It is the directness of this knowledge that differentiates it from all other a posteriori knowledge. All knowledge by acquaintance is a posteriori, but not all a posteriori knowledge is knowledge by acquaintance. My awareness of pain is knowledge by acquaintance, yet when I infer that “something is causing me pain,” this belief is propositional. Russell’s distinction between knowledge by acquaintance and propositional knowledge, if accurate, has important implications in epistemology. It shows that inference is used even in cases of beliefs that people think are obvious: ordinary beliefs based on perception. Russell thought that one can only have knowledge by acquaintance of one’s sensations and cannot have direct awareness of the objects that could be the cause of those sensations. This is a significant point. When I see the bird on a branch outside my office window, I am not immediately aware of the bird itself. Rather, I am directly aware of my perceptual experience of the bird—what philosophers call sense data. Sense data are sensations gained from perceptual experience; they are the raw data obtained through the senses (seeing, smelling, feeling, etc.). One’s perceptual experience is of sense data, not of the objects that could be causing that sense data. People infer the existence of external objects that they believe cause their perceptual experiences. Russell’s view implies that people always use reasoning to access the external world. I have knowledge by acquaintance of my perceptual experience of seeing a bird; I then infer ever so quickly (and often unconsciously) that there is a bird on the branch, which is propositional knowledge. Not all philosophers think that experience of the external world is mediated through sense data. Some philosophers contend that people can directly perceive objects in the external world. But Russell’s theory introduces an important possibility in epistemological thinking: that there is a gap between one’s experience of the world and the world itself. This potential gap opens up the possibility for error. The gap between experience and the world is used by some thinkers to argue that knowledge of the external world is impossible. Table 7.1 summarizes the types of knowledge discussed in this section. Type Description Examples Propositional knowledge Knowledge of propositions or statements; knowledge of facts Examples are infinite: “I know that…” the Earth is round, two is an even number, lions are carnivores, grass is green, etc. Procedural knowledge “Know-how”; understanding how to perform some task or procedure Knowing how to ride a bicycle, do a cartwheel, knit, fix a flat tire, dribble a basketball, plant a tree, etc. Knowledge by acquaintance Knowledge gained from direct experience Perception of physical sensations, such as pain, heat, cold, hunger; important to differentiate between the knowledge by acquaintance that is the sensation (e.g., a physical sensation of feeling cold) and related inferences, such as “the air temperature must be dropping,” which is propositional knowledge. Table 7.1 - Types of Knowledge

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