Step 1: Understanding the Concept:
The question relates to the legal principle of "conclusive proof" under the Indian Evidence Act, specifically in the context of Section 112, which deals with the presumption of legitimacy of a child born during a valid marriage. A conclusive presumption is a rule of law that a court is bound to accept and cannot be rebutted by any counter-evidence. The question asks for the case that established the foundational principle behind the rigidity of such presumptions.
Step 2: Detailed Explanation:
Section 112 of the Evidence Act creates a conclusive presumption that a child born during the continuance of a valid marriage between his mother and any man is the legitimate son of that man. The only way to rebut this presumption is by proving that the parties to the marriage had no access to each other at any time when he could have been begotten.
The statement in the question reflects a strict interpretation of this rule, suggesting that even scientific evidence like a DNA test cannot be used to disprove paternity if the condition of "non-access" is not met. While the jurisprudence on the conflict between DNA evidence and Section 112 has evolved over time (with later judgments allowing DNA tests in certain situations), the foundational principle for the strength of a "conclusive presumption" comes from the case of Somwanti & Co. v. State of Punjab, AIR 1963 SC 151.
In Somwanti, the Supreme Court explained the meaning of "conclusive proof" as defined in Section 4 of the Evidence Act. It held that where one fact is declared by the Act to be conclusive proof of another, the Court, upon proof of the first fact, must regard the second as proved and shall not allow evidence to be given for the purpose of disproving it.
Although \textit{Somwanti} was not a case about paternity or DNA, it is the landmark authority that explains the legal mechanism of conclusive proof, which is the basis of Section 112. Therefore, it provides the legal reasoning for the statement in the question.
Step 3: Final Answer:
The foundational principle regarding the irrebuttable nature of a "conclusive proof" was authoritatively laid down in Somwant v. State of Punjab, AIR 1963-SC 151.