To a general audience and the people that take the time to analyze, critique, and laud works of art such as albums, movies, books, and video games, those four mediums are self-evidently compartmentalized, easily understood as their own standalone outfit to be judged as a complete work. Even in the case of sequels, trilogies, and series as a whole, it is still more than reasonable to separate out each individual work of those types in their analyses. But for the case of television, there's a lot more that's up to personal interpretation.
Those other artistic mediums' proclivities, more specifically their contextual affects, can only concretely stem from both the context of a series, along with—in a perhaps wider range of art—the evolution of the respective creators' visions, production, and styles. And in their production and release, those two facets are most of what is considered. But TV shows are much more mobile, vulnerable, and less grounded in nearly every aspect, subject to change at the whim of their directors' and writers' alterations to a given show's original formulae. They are more than within their right to change the original narrative seen at the production's onset by even its second episode, let alone the second season.
Compounding all of that is the time requirements to produce a single season of a TV show, which can add simply the passage of time as a crucial change, within that already huge set of factors warping the intentions and final facets of the following season, and the show as a whole.
Within that framework then, of realizing all of the minute differences that can alter everything a TV show was originally made to represent and display across seasons' shifts, it is only reasonable to consider seasons as the closest thing to a standalone, easily digestible section of a broader work. Even if the completed TV show is in itself deserving of a wider recognition of how all of its nested seasons played and worked together, that reason mirrors exactly why it is valuable to unpack each season on its own. Because even when taken on their own, each subsequent season owes a plurality of its effect and significance on its preceding episodes, even in the case of anthological works.
In effect, it operates like a reply-all email, requiring the perceived input of all previous episodes and context to warrant analyzing that piece of its puzzle. That is what allows a review any meaning or purpose, as attempting to constrain what makes the quality of a second season better without discussing what changed, what was altered in contrast to its first season, is pointless. There has to be some addressing of the preceding art to understand and contextualize that current art.
So therefore, despite it making a lot of sense to review a completed tv show as one robust work in its own right—ignoring all the cases of canceled TV shows repeatedly being resurrected after more than a couple intended series finales—it makes much more sense to treat each season as a cumulative addition, along with being its own fundamental entity to analyze and understand.
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