One of the synonyms for the phrase "rational ground" is "logical basis". This implies that a certain concept, idea or argument has been formulated using sound reasoning, facts and evidence. Another synonym is "reasonable justification", which suggests that a particular decision or action is being made based on valid and sensible reasons. "Sound rationale" is another possible synonym, which emphasizes the importance of having a strong and well-reasoned explanation. Ultimately, using synonyms for "rational ground" highlights the significance of logic and reason in the decision-making process, as well as the importance of being able to provide a clear and justifiable explanation for one's actions and beliefs.