While logical fallacies certainly exist, there are a few so- called fallacies that are valid despite their label:
Appeal to Authority. This should be obvious, as anyone who recites from a list of “fallacies” are themselves appealing to the Authority the list represents, to back up their claim that your opinion is invalid. In fact, their appeal to the Authority of the list of logical fallacies, is an example of self- contradiction, i.e. another fallacy.
Slippery slope: we all know the quote “those who ignore history are doomed to repeat it”, and of one important event in history particularly, we keep Remembrance Day in our hearts “Lest we forget” the chain of events that led the world into the Second World War in the first place. When we see an elected leader demonstrating some of the same behaviours as one who manipulated his Authority to gain the powers of a Dictator- i.e. suspending elections, declaring Martial Law, drafting the Enabling Act, etc. and want to caution others where this is leading, “slippery slope” remains a valid argument- NOT a fallacy.
Another example where ‘slippery slope’ might be used is the argument that decriminalising homosexuality (illegal under Canadian Law under “buggery” until 1993) would result in other morality based sex crimes (e.g. pedophilism, bestiality, etc) become legal. It isn’t hard to understand how there might be some cause for concern- in Elizabethan time certain sexual acts we find ‘immoral’ and in fact criminal today were legal, ‘moral’, and common, as in the case of the character ‘Juliet’ in Shakespeare’s play, i.e. arranged marriages between cousins where the bride is under 14 years old . The argument is that if we accept that morality is subjective and therefore we cannot make certain behaviours, e.g. homosexuality, baring female breasts in public, etc. criminal, where DO we draw the line in determining what is justifiably illegal for public safety and what is illegal on the basis of current societal mores alone. The line is indeed distinct to each person, but when the attempt is made to draw the line, it becomes clear that it is seldom that universal consensus on WHERE to draw the line is impossible. What IS clear, is that sexual behaviours are trending towards relaxing the regulations, if not complete removal, and while some would applaud the removal of ‘unjust laws’, others would feel cause for alarm, fearing a loss of personal security.