The word "之所以" is formed through the historical fusion and grammaticalization of three distinct classical Chinese function words: the possessive/linking particle 之, the nominalizing particle 所, and the preposition 以 (meaning "by means of" or "because of"). Over time, this specific combination solidified into a fixed, inseparable compound that functions as a single conjunctive unit. Its formation is essentially a fossilized syntactic phrase from classical Chinese that has lexicalized in modern usage to serve as a dedicated marker for introducing a cause or reason, with its internal components losing their individual syntactic freedom within the structure.