Formal fallacies concern faulty inferences found in deductive reasoning. Denying the antecedent is one type of formal fallacy, for example, "If Othello is a bachelor, then he is male. Othello is not a bachelor. Therefore, Othello is not male". Informal fallacies, on the other hand, apply to all types of reasoning. The source of their flaw is to be found in the content or the context of the argument. This is often caused by ambiguous or vague expressions in natural language, as in "Feathers are light. What is light cannot be dark. Therefore, feathers cannot be dark". An important aspect of fallacies is that they seem to be rationally compelling on the first look and thereby seduce people into accepting and committing them. Whether an act of reasoning constitutes a fallacy does not depend on whether the premises are true or false but on their relation to the conclusion and, in some cases, on the context. Concept formation Concepts are general notions that constitute the fundamental building blocks of thought. They are rules that govern how objects are sorted into different classes. A person can only think about a proposition if they possess the concepts involved in this proposition. For example, the proposition "wombats are animals" involves the concepts "wombat" and "animal". Someone who does not possess the concept "wombat" may still be able to read the sentence but cannot entertain the corresponding proposition. Concept formation is a form of thinking in which new concepts are acquired. It involves becoming familiar with the characteristic features shared by all instances of the corresponding type of entity and developing the ability to identify positive and negative cases. This process usually corresponds to learning the meaning of the word associated with the type in question. There are various theories concerning how concepts and concept possession are to be understood. According to one popular view, concepts are to be understood in terms of abilities. On this view, two central aspects characterize concept possession: the ability to discriminate between positive and negative cases and the ability to draw inferences from this concept to related concepts. Concept formation corresponds to acquiring these abilities. It has been suggested that animals are also able to learn concepts to some extent, due to their ability to discriminate between different types of situations and to adjust their behavior accordingly. Problem solving In the case of problem solving, thinking aims at