Phil Critical Thinking. Southern Illinois University Edwardsville. Deductive Concepts. Deduction vs. Deductive Form: The premises are intended to provide conclusive reasons or proof of the conclusion.
Inductive Form: The premises are intended to provide compelling but not conclusive reasons for the conclusion. Definitions these definitions are just two different ways of saying the same thing. Validity in the technical sense just defined applies only to arguments, never to individual claims.
If some argument is valid, then every argument with the same structure is also valid. If an argument has one or more false premises or it is not valid, then the argument is not sound. My pet has four paws. Therefore, my pet is a dog.
Plato — BC believed that all things are divided into the visible and the intelligible. Intelligible things can be known through deduction with observation being of secondary importance to reasoning and are true knowledge. Aristotle took an inductive approach, emphasizing the need for observations to support knowledge.
He believed that we can reason only from discernable phenomena. From there, we use logic to infer causes. Debate about reasoning remained much the same until the time of Isaac Newton. In his Principia, Newton outlined four rules for reasoning in the scientific method :. In , philosopher John Stuart Mill published A System of Logic , which further refined our understanding of reasoning.
Mill believed that science should be based on a search for regularities among events. If a regularity is consistent, it can be considered a law. Mill described five methods for identifying causes by noting regularities. These methods are still used today:. Karl Popper was the next theorist to make a serious contribution to the study of reasoning. Popper is well known for his focus on disconfirming evidence and disproving hypotheses.
Beginning with a hypothesis, we use deductive reasoning to make predictions. A hypothesis will be based on a theory — a set of independent and dependent statements. If the predictions are true, the theory is true, and vice versa. This process requires vigorous testing to identify any anomalies, and Popper does not accept theories that cannot be physically tested.
Any phenomenon not present in tests cannot be the foundation of a theory, according to Popper. The phenomenon must also be consistent and reproducible. Science is always changing as more hypotheses are modified or disproved and we inch closer to the truth. No discussion of logic is complete without a refresher course in the difference between inductive and deductive reasoning. By its strictest definition, inductive reasoning proves a general principle—your idea worth spreading—by highlighting a group of specific events, trends, or observations.
In contrast, deductive reasoning builds up to a specific principle—again, your idea worth spreading—through a chain of increasingly narrow statements. Logic is an incredibly important skill, and because we use it so often in everyday life, we benefit by clarifying the methods we use to draw conclusions.
Knowing what makes an argument sound is valuable for making decisions and understanding how the world works. It helps us to spot people who are deliberately misleading us through unsound arguments.
Understanding reasoning is also helpful for avoiding fallacies and for negotiating. Read Next. Mental Models Reading Time: 12 minutes. As my friend Peter Kaufman says : What are the three largest, most relevant sample sizes for identifying universal principles?
Deductive and inductive reasoning are both based on evidence. Several types of evidence are used in reasoning to point to a truth: Direct or experimental evidence — This relies on observations and experiments, which should be repeatable with consistent results. Anecdotal or circumstantial evidence — Overreliance on anecdotal evidence can be a logical fallacy because it is based on the assumption that two coexisting factors are linked even though alternative explanations have not been explored.
The main use of anecdotal evidence is for forming hypotheses which can then be tested with experimental evidence. By definition of validity. A valid argument cannot have all true premises but a false conclusion. Arguments are not the kinds of things that can be true or false. Only individual statements have a truth value, and arguments are sets of statements. A valid argument can have all false premises and a true conclusion.
P1: If Lassie is a frog, then she is a mammal. P2: Lassie is a frog. C: Lassie is a mammal. All invalid arguments are such that it is possible for them to have true premises and a false conclusion; and some invalid arguments actually do have all true premises and a false conclusion. Validity is a necessary condition for being sound. A valid argument can have a true conclusion and false premises see 11 ; and if an argument does not have all true premises, then it is not sound.
By definition, a valid argument cannot have a false conclusion and all true premises. So if a valid argument has a false conclusion it must have some false premise. It just means succeeding in establishing conclusive support for its conclusion. Of course, the premises of this argument are false. But claiming that an argument is valid is not to claim that the premises are true.
Validity is about succeeding in providing conclusive support for the conclusion, if the premises were true. So if the conclusion is unlikely to be true when the premises are true, then the argument is weak.
Game over. The answer to this question is contextual. As a lecturer, my standards are very strict. My goal is to make sure that you learn from your mistakes. I need to change my standards there. So establishing that an argument is strong in court is quite demanding.
We want to minimise the mistakes we make. Madison is a vegetarian. Therefore, Madison is probably healthy. If the premises are true, what are the chances that Madison is healthy? She might not be. Hence, this argument is strong. Want to keep learning? This content is taken from The University of Auckland online course,. This content is taken from The University of Auckland online course. Share this post. See other articles from this course. This article is from the online course:. Join Now.
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