Nearly all debates in epistemology are in some way related to knowledge. Most generally, "knowledge" is a familiarity, awareness, or understanding of someone or something, which might include facts (propositional knowledge), skills (procedural knowledge), or objects (acquaintance knowledge). Philosophers tend to draw an important distinction between three different senses of "knowing" something: "knowing that" (knowing the truth of propositions), "knowing how" (understanding how to perform certain actions), and "knowing by acquaintance" (directly perceiving an object, being familiar with it, or otherwise coming into contact with it). Epistemology is primarily concerned with the first of these forms of knowledge, propositional knowledge. All three senses of "knowing" can be seen in our ordinary use of the word. In mathematics, you can know 2 + 2 = 4, but there is also knowing to add two numbers, and knowing a (e.g., knowing other persons, or knowing oneself), (e.g., one's hometown), (e.g., cars), or (e.g., addition). While these distinctions are not explicit in English, they are explicitly made in other languages, including French, Portuguese, Spanish, Romanian, German and Dutch (although some languages related to English have been said to retain these verbs, such as Scots). The theoretical interpretation and significance of these linguistic issues remains controversial.