Logic and Critical Thinking in a Secular Society

Logic and Critical Thinking Secular Society, 15th September 2015

Formal Logic

Informal Logic

Propositions A proposition is a statement of fact which bears a truth value, usually true or false.

Propositions Which of the following are propositions? Paris is the Capital of France It is snowing outside Stop talking so loudly! 4 + 16 = 21 Are you hungry? 你好 Twasbryllyg, and ye slythytoves did gyre and gymble in ye wabe

Belief Belief is a type of mental attitude which we hold with respect to a proposition and its truth value. We may think the proposition is true “believe it”, think it is false “disbelieve it”, or have no opinion “neither believe nor disbelieve”.

Arguments In philosophy, an argument consists of a series of propositions called premises which, taken together, are intended to establish the truth of another proposition called the conclusion.

Arguments Premise 1: All atheists are immoral Premise 2: I (James Fodor) am an atheist Conclusion: I am immoral

Arguments Premise 1: God is a being than which no greater can be conceived (GCB) Premise 2: If GCB does not exist, then I can conceive of a still greater being, namely one that exists Premise 3: But this is absurd, for I cannot conceive of a being greater than a GCB Conclusion: Therefore a GCB, namely God, exists

Arguments How can an atheist justify morality in the absence of a God? On what basis can they claim that their subjective sense of goodness is superior to anyone else's? What place can a concept like goodness have in a purely naturalistic, material universe?

Validity An argument is valid if its conclusion follows logically from its premises. Premise 1: Richard Dawkins is a man Premise 2: All men are mortal Conclusion: Richard Dawkins is mortal

Validity Premise 1: If Jesus performed many miracles, he must be the Son of God Premise 2: Jesus performed many miracles Conclusion: Jesus is the Son of God

Validity Premise 1: There are many different concepts of God in different religions Premise 2: Most of these ideas about God don’t many any rational sense Premise 3: No objective evidence for God’s existence has ever been found Conclusion: God does not exist

Soundness An argument is sound if it is valid (its conclusion follows from its premises), and also all its premises are true.

Soundness Premise 1: There is no credible historical evidence that Jesus existed as a real person Premise 2: If Jesus did not exist as a real person, Christianity is false Conclusion: Christianity is false

Soundness Premise 1: Greenhouse gas emissions are a major cause of global warming Premise 2: A carbon tax would reduce greenhouse gas emissions Conclusion: We should adopt a carbon tax

Logical Fallacies A logical fallacy is a form of reasoning or type of argument which can appear to be valid, when in fact it is not. Note: examples to follow are for illustration only. No endorsement is made regarding their premises or conclusions. Nor do I claim that the arguments could not be reframed in a way to make them valid or sound.

Begging the Question Using one’s desired conclusion, or a statement which is dependent on that conclusion, as a premise in one’s argument. Example: ‘God cannot exist, for to be that intelligent and powerful his brain (whatever form it was in) would have to be larger than the known universe. Its just not possible to fit infinite processing power and storage into finite space’.

Equivocation The misleading or confusing use of a single word or phrase to mean different things in different parts of an argument. Example: ‘Once a zygote is formed, the cell has a full set of human chromosomes, and so is a human being. Since human beings have a right to life, from fertilization on the embryo also has a right to life.’

Fallacy of Composition Falsely concluding that a property obtains of a whole on the basis that the property obtains of the parts of the whole. Example: ‘In an atheistic world, humans are nothing more than atoms moving about under the laws of physics. Atoms don’t have any moral value – they just exist. Thus, under an atheistic worldview humans have no more moral status than puddles or slime or piles of rocks.’

Fallacy of the Single Cause An argument which infers that because some A is caused by B, that therefore C cannot also be a cause of A. Example: ‘Guns don’t kill people. People kill people.’ ‘Religious experiences can be shown to be the result of activity in particular regions of the brain, therefore they are not caused by any spiritual intervention.’

Appeal to Nature An argument which infers the goodness of some act or outcome on the basis that it is natural. Example: ‘Homosexuality is found throughout the animal kingdom and across many human cultures. Homosexual attraction is largely a product of genes and not social environment. We see that homosexuality is natural and so not immoral.’

Correlation and Causation It is false to assert that because to things are found to co-occur, that therefore one is the cause of the other. Example: ‘Under Party X, there was a recession and massive unemployment, while under Party Y, the economy grew and unemployment shrank. Therefore Party Y has superior economic management skills to Party X.’

Reification Fallacy Appealing to an abstraction as if it were a real, concrete entity. Example: ‘Islam is an archaic and barbaric religion. It teaches the subjugation of women, is anti-democratic, and acts to oppose scientific progress.’ Other candidates: capitalism, Christianity, feminism, Atheism, neo-Liberalism, Socialism, nature

The Fallacy Fallacy Inferring the falsity of the conclusion of an argument on the basis that the argument is fallacious. Example: If I were to call someone an idiot and their argument stupid, they may respond by saying that I was using an ad hominem attack, and thus my argument is invalid. This may be true, but does not show that my conclusion is false.

Conditionals A conditional is a statement of the form: “If X then Y”. In philosophy, conditional statements are only false if Y is false while X is true. Example: “If cigarettes aren’t harmful for health, we shouldn’t restrict smoking”

Red Herrings Red herrings are statement or details that aren’t by themselves directly relevant, but often appear to be, or are made out to be. Example 1: Homeopathy website http://www.homeopathycenter.org/learn-about-homeopathy Example 2: “Say no to $100,000 degrees”

Appeals to Authority Appeals to authority are not necessarily fallacious. It depends how they are used. Invalid example: Atheist thinkers understand that objective morality is impossible under an naturalistic worldview, for example Nietzsche says... Valid example: 98% of climate scientists agree that global warming is occurring, therefore it is reasonable to believe this is so.

Logic and Critical Thinking in a Secular Society