【Valid】 , 【sound】 ,  【cogent】 ,  【convincing】 ,  【compelling】 ,  【telling】  can all be applied directly or indirectly to arguments, reasons, principles, or processes of thought or to their presentation and mean having or manifesting the power to impress themselves on others as right and well-grounded.

【Valid】  and  【sound】  both imply that the power is inherent in the rationality or logicality of the thought apart from its presentation.

Something is  【valid】 against which no objections can be maintained, because it conforms strictly to the law or regulations (as of the state or the church) or because it is supported or justified by facts and correct reasoning or, less often, because it is fully in accordance with claims or promises made for it and is entirely effectual or efficacious.

A person or a thing is  【sound】 that is free from error or fault in his or its processes of thought and that avoids fallacies, insufficient evidence, hasty conclusions, or superficiality. The term not only suggests flawlessness in reasoning but solidity in the grounds upon which this reasoning is based.

Something 【cogent】  or  【convincing】  or  【compelling】  commands mental assent.

But  【cogent】  tends to stress a power or force resident in the argument or reasoning that makes it conclusive,  【convincing】  suggests a power to overcome doubt, opposition, or reluctance to accept, and  【compelling】 calls particular attention to the substantial nature of the objective evidence.

Something  【telling】 produces at once the desired effect; frequently the term implies the 【compelling】 of assent but it seldom directly suggests soundness or cogency though, in general, it does not deny the existence of these qualities.

The term is often applied to words, phrases, tones of expression, or rhetorical methods which convince, persuade, or win admiration and support because of their pertinency, their suitability, or their forcibleness.