A Morton's fork is a specious piece of reasoning in which contradictory observations lead to the same conclusion. It is said to have originated with the collecting of taxes by John Morton, Archbishop of Canterbury in the late 15th century, who held that a man who was living modestly must be saving money and could therefore afford taxes, whereas if he was living extravagantly then he was obviously rich and could still afford them.
In some instances, such as Morton's original use of the fallacy, it may be that one of the two observations is likely valid but the other is pure sophistry: evidence of possessing wealth may be genuinely relevant to having a source of taxable income.
In other cases, it may be that neither observation can properly be relied upon to support the conclusion. For example, asserting that a person suspected of a crime who is acting nervously must have something to feel guilty about, while a person who acts calmly and confidently must be practiced or skilled at hiding his guilt. Either observation therefore has little, if any, probative value as each could equally be evidence of the inverse situation.