2.7. Summary

Forming arguments is a fundamental aspect of critical thinking, particularly in psychology and other disciplines, as it enables systematic evaluation of theories, evidence, and complex issues. An argument, in this context, is defined as a set of propositions where premises provide reasons to support a conclusion. Unlike casual disagreements, effective arguments rely on logical reasoning and evidence to persuade. Propositions, the foundation of arguments, are declarative statements that can be true or false. They serve specific roles, with premises supporting conclusions through inference, a logical connection between ideas. For instance, the premises “All humans are mortal” and “Socrates is human” lead to the conclusion “Socrates is mortal”. Critical thinking involves constructing such arguments and critically evaluating the validity of premises and their relationship to conclusions, fostering informed and reasoned decision-making.

Propositions, as the building blocks of arguments, assert claims about the world that can be evaluated for truth or falsity. Effective arguments rely on credible premises, often assumed to be self-evident or justified separately, to strengthen their conclusions. Propositions may be empirical, based on observable evidence, or rational, derived from reasoning about concepts. Critical thinking requires analysing arguments by identifying premises and conclusions, evaluating their truth, and assessing how well they support one another. Additionally, recognising cognitive biases, fostering humility, and practising structured reasoning are crucial for countering flaws in human cognition and navigating an increasingly complex and manipulative information landscape. These skills enable clearer thinking, better decision-making, and meaningful contributions to society.

Implicit premises, or unstated assumptions in arguments, can influence validity, especially when they are controversial or false. To evaluate arguments effectively, implicit premises should be made explicit, allowing for proper assessment of their truth and their connection to the conclusion. For instance, normative conclusions, which are statements about what should or ought to happen, require at least one normative premise to be valid, as descriptive premises alone cannot justify normative claims. Failing to include these premises leads to logical fallacies, such as deriving “ought” from “is”. Reconstructing arguments with all necessary premises, especially for normative conclusions, ensures clarity and validity in reasoning.

Deductive arguments aim to guarantee the truth of their conclusions through the logical structure of their premises. A valid deductive argument ensures that if all premises are true, the conclusion must also be true, regardless of the premises’ actual truth. Validity pertains to an argument’s structure rather than its content, as seen in examples like “If all cats can fly and Whiskers is a cat, then Whiskers can fly.” However, validity alone does not make an argument sound; the premises must also be true. Understanding common valid patterns, such as Modus Ponens, Modus Tollens, disjunctive syllogism, and hypothetical syllogism, alongside recognising formal fallacies like affirming the consequent or denying the antecedent, is essential for effectively evaluating deductive arguments.

Non-deductive arguments, including probabilistic reasoning, enumerative inferences, arguments from samples, analogies, causal reasoning, and inferences to the best explanation, differ from deductive arguments in that they do not guarantee their conclusions but offer varying degrees of likelihood. For example, Sherlock Holmes’ reasoning often involves inference to the best explanation, selecting the most plausible cause for observed phenomena while considering alternatives. The strength of non-deductive arguments depends on how well their premises support the conclusion and can vary in degree, unlike the binary validity of deductive arguments. Evaluating such arguments requires assessing factors like sample size, randomness, relevance, and plausibility. While these arguments are inherently open to error, they remain essential for reasoned judgement in addressing uncertainties in everyday reasoning and scientific investigation.

Evaluating arguments involves analysing their structure and the truth of their premises to determine whether they are sound (deductive arguments) or cogent (non-deductive arguments). A sound argument is valid and has true premises, ensuring a true conclusion. Conversely, a cogent argument is strong and has true premises, making its conclusion likely but not guaranteed. Assessing the truth of premises is essential, as it determines whether statements accurately reflect reality. Non-deductive arguments, while cogent, can lead to false conclusions since their premises only make the conclusion probable. Therefore, beyond evaluating cogency, it is crucial to consider external evidence that might challenge the conclusion, highlighting the nuanced relationship between cogency and truth.