Essentiality without Necessity

It is widely accepted that if a property is essential then it is necessary. Against this I present numerous counterexamples from biology and chemistry, which fall into two groups: (I) A property is essential to a genus or species, yet some instances of this genus or species do not have this essential property. (II) A property is essential to a genus, yet some species of this genus do not have this essential property. I discuss and reject four minor objections. Then I discuss in depth whether a distinction between constitutive essence and consequential essence is able to handle these counterexamples. I conclude that this distinction is better put as one between (1) the essence, which is necessary, and (2) the essential properties, which are not formally necessary. An essence of an object X is the substantial universal expressed by its real definition. An object X has a property P essentially iff the property P is explanatory and non-trivial, and P follows from the essence of X.