Look up cluster concept in wikipedia. With a cluster concept, there is a cluster of features that define the concept, no one of which is essential. But when too few are present, most observers will say, "That really isn't a kayak. The guy pedals with his feet to move."
Things defined by cluster concepts always cause a bit of controversy. It's unavoidable. But as Kierkegaard said of the Danish State Church, "There's no good saying it's a poor sort of a horse. It isn't a horse at all, it's a cow!"
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