Final answer:
In philosophy, a 'why' question can no longer be answered when it leads to an infinite regress.
Step-by-step explanation:
In philosophy, the concept of an infinite regress refers to a sequence of causes and effects that goes on infinitely without a starting point. In the context of the question, a 'why' question can no longer be answered when it leads to an infinite regress.
For example, if someone asks 'Why did A cause B?' and the answer is 'Because C caused A', and then if we ask 'Why did C cause A?' and the answer is 'Because D caused C', and so on, this chain of causation can continue endlessly without reaching a definitive answer. At this point, the 'why' question can no longer be answered because it leads to an infinite regress.
Therefore, in philosophy, when a 'why' question leads to an infinite regress, it reaches a point where it can no longer be answered.