Why is mutual intelligibility not a perfect test for deciding whether speech varieties are dialects or separate languages?

Mutual intelligibility is not a perfect test for distinguishing between dialects and separate languages because the degree of intercomprehensibility exists along a relative scale rather than a clear-cut binary.12

While dialects of the same language are generally expected to be mutually intelligible, this intelligibility is rarely absolute. Conversely, speakers of closely related languages can often communicate to a significant extent using their respective mother tongues. Because there is no objective, quantifiable threshold for how much comprehension is required to classify speech varieties as dialects versus distinct languages, the final cutoff point is often arbitrary.12

Furthermore, sociopolitical factors frequently override linguistic criteria. Varieties that are viewed as dialects under one set of historical or political circumstances may be reclassified as separate languages following shifts in social or national identity, as seen in the case of Serbo-Croatian after the conflicts in the Balkans. Consequently, the distinction between a dialect and a language is often determined more by social and political context than by the inherent ability of speakers to understand one another.1