Basic Logic Revision PHI1024F Logic Revision
Foundations of Informal Logic
Informal logic focuses on the structure of arguments, which are defined as packages of propositions consisting of premises and conclusions. Premises provide the reasons that support a conclusion, while the conclusion is the statement serving as the goal of the argument. In English, premises are often identified by indicator words like "for", "because", or "since", while conclusions are marked by terms such as "therefore", "thus", "hence", and "consequently".
Deductive Reasoning and Evaluation
Deductive arguments are structured to preserve truth, meaning the conclusions follow necessarily from the premises. These arguments are evaluated based on validity and soundness. Validity refers strictly to the argument's form; an argument is valid if its structure ensures that the conclusion is true whenever the premises are assumed to be true. A standard valid form is: If , then . . Therefore . Soundness is achieved when an argument is valid and its premises are factually true in reality.
Deductive Fallacies
Fallacies in deductive reasoning are structural failures that resemble valid argument forms but are logically invalid. A primary example is affirming the consequent, which takes the form: If then . . Therefore . These "fake lookalikes" fail to preserve truth despite their superficial similarity to valid deductive structures.
Inductive Reasoning and Evaluation
Inductive reasoning establishes likely or probable conclusions based on data rather than achieving absolute certainty. It is measured by strength and cogency instead of validity and soundness. An argument is strong if the truth of the premises makes the conclusion probably true, such as Hume's argument regarding the sun rising. It is considered cogent if it is strong and the premises are actually true. Inductive reasoning typically utilizes probabilistic language like "most" or "usually" rather than the universal terms like "all" or "always" found in deductive logic.