A recently publicized dispute in Oak Park, Mich., pits a homeowner who has planted a vegetable garden in her front yard against her neighbors and the city (which initially dug up the spot in question for sewer repairs). City code stipulates that front-yard vegetation consist of “suitable live plant material”. The city’s Technical and Planning Director maintains that “suitable means common”, and, citing the fact that front-yard vegetable gardens are uncommon in Oak Park, finds the homeowner in violation of city code. The homeowner and the city have vowed to fight the matter out in court, with the homeowner facing up to 93 days in jail if convicted.
The city’s position apparently rests on the claim that suitable and common are near-synonyms. In fact, it is a simple and instructive exercise in linguistic semantics to show that suitable and common have markedly different meanings. The point can be demonstrated in at least two ways: by considering the basic truth conditions of the adjectives, and by considering the different ways in which their meanings involve judgment or opinion.
First, the basics: suitable and common fail the most fundamental test for synonymy. It is possible for something to be suitable without being common, and vice versa. To illustrate, imagine two different kinds of front-yard vegetation: (i) a rare and exotic varietal of grass, and (ii) dandelions. The special grass requires less water than ordinary grass but is otherwise indistinguishable from it; it is non-invasive, carries no exotic plant illnesses, etc. Dandelions are just ordinary dandelions, provoking the standard prescriptivist lawn-care anxiety and ire. Now assume, for the sake of argument, that just one home in Oak Park has the special grass, but that 35% of the city’s homes have dandelions in their front yards. In this scenario, we would say that the special grass is suitable but not common; conversely, we would say that dandelions are common (assuming that 35% saturation meets the relevant threshold; if not, adjust it upward as you see fit) but not suitable. Clearly, then, suitable and common have different truth conditions: they make different claims about what the world must be like in order for a sentence containing them to be true.
Beyond this basic truth-conditional difference, the meaning of suitable depends on personal judgment and opinion in a way that the meaning of common does not. To be sure, some opinion is required in the ordinary (or, in linguistic terminology, “positive”) use of common: as alluded to above, speakers may disagree on just how widespread something must be in order to count as common. But this is a very different sort of disagreement from that involved in the use of suitable. To see this, we must look at these adjectives’ behavior in the comparative degree. Though speakers’ opinions may vary about what counts as common, statements of relative commonness should be stable across speakers. Imagine as above that 35% of front yards have dandelions; now imagine that John believes that something counts as common if it is found in a third of the city’s front yards, but Mary believes that it must be found in at least half of the city’s front yards to count as common. In this scenario, John will judge dandelions to be common in Oak Park, and Mary will not. But they will both agree that dandelions are more common than the special grass. Unlike plain commonness, relative commonness does not require the speaker to have a particular subjective threshold in mind; rather, it involves an objective comparison. The same cannot be said for suitable. Even in the comparative degree, judgments will vary across speakers: since suitability is fundamentally a matter of personal judgment and opinion, people can disagree about whether one thing is more suitable for a particular purpose than another. Informally speaking, common is subjective, but more common is objective; by contrast, suitable is subjective, and more suitable is also subjective.
All of this is not to say that there isn’t a grain of truth in the city’s argument. In a law-abiding society, things that are unsuitable will be uncommon. And, on one reading of the sentence, things that are suitable will be common. Crucially, though, this means that one will commonly encounter things that are suitable; as I have attempted to illustrate above, it does not necessarily mean that all suitable things will be common. Unfortunately, specious linguistic arguments like those offered by the city of Oak Park, though not suitable, are all too common.