"Grounds for belief" refers to evidence or justification that supports a belief or opinion. There are several synonyms for this phrase, including "rationale," "justification," "basis," "reasoning," and "evidence." Each of these words has a slightly different nuance, but they all represent the idea that beliefs should be supported by some form of logic or evidence. Without proper grounds for belief, opinions can be arbitrary and unsupported, leading to erroneous conclusions and flawed arguments. Therefore, it is important to carefully consider the grounds for belief when forming opinions or making important decisions.