Essence as a Concept
In philosophy, essence is the property or set of properties that make an entity or substance what it fundamentally is, and which it has by necessity, and without which it loses its identity. The concept originates rigorously with Aristotle (although it can also be found in Plato), who used the Greek expression to ti esti (literally meaning “the what it is“). 
This is the philosophy behind characterizing a person’s essence, also known as one’s temperament, that can be described by higher order traits or dimensions that are further divided into sub-traits or facets. Some of these interesting features or tendencies are driven directly by our unique physiological makeup, some just hint to it and are much further up the chain, while others correlate with other facets, giving clue that one is tangled with the other, and some may bear no such relationship.
To be sure, it’s a tough task to etch out a particular tendency such that it has a stable and definable input response, say stimulating an infant with a light shined to its eyes (the input) to measure a response (the output), since the outputs will change when the situation changes. But we can still capture a definable quality as described by the behavior we observe, and the idea that it’s impossible to get at the “essence” of these observations has been wrongly expressed throughout our philosophical history.
Niels Bohr was skeptical about our ability to grasp whatever “hidden whole” (the physiological process) lay behind what was observed (the trait’s behavior), even though he agreed it was necessary to use words as conceptual aids to describe the invisible processes. But these words were conjectures to help understanding. 
Essence as an Example
To locate the essence of a behavior, means we need to get as close as possible to the physiological response that is shaped by genetics and unique to a person or class of people. So for a familiar example, the temperament – a genetically influenced physiological response – called inhibition to the unfamiliar, changes upon situation, say shy when the situation is people, say timid when it is an unfamiliar situation, and finicky when with unfamiliar food. So inhibition (the “hidden whole”) is the essence of the traits labeled as shy, timid and finicky since its closest to one’s physiology.
To side with Einstein, however, is to commit the error of awarding a trait [my insertion] a Platonic reality-a thing in itself-that is a fiction. Hilary Putnam (1995) argues that when we talk of “ding an sich” [a thing as it is in itself, not understood through human perception]” we do not know what we are talking about. 
And because we know the causes (the essence) of the observable phenomena, it is no longer a platonic reality. In this example, it’s the physiological response to a stimulus applied to infants to determine how “reactive” they are. Developmental psychologist Jerome Kagan found that 20% of infants, always in that proportion if sample size large, “showed limbic activity combined with distress to visual, auditory, and olfactory stimuli”, “EEG activation on the right frontal area (where negative affect occurs) under resting conditions while most infants show activation in the left frontal area”.
Essence as Elusiveness
The trait of inhibitedness, however, may not be the fundamental trait since “high sensory-processing sensitivity” of brain regions and the nervous system exist in individuals that are labeled as “high-reactive”, which results in sensitivity to subtle stimuli, cues, and novelty, as well as being easily overstimulated. And people with this trait can appear to be shy because they often hesitate and are thus described by Dr. Aron as “geared to pause, inspect and reflect” .
An analogy may be helpful here to illustrate why “sensitivity” is a better descriptor than “inhibitedness” just as “skin-cancer-proneness” is not as generally useful a descriptor of a blonde, blue-eyed person’s major physical traits as “fair” or “fair complexioned” would be” .
The importance of sensitivity or inhibitedness (which can result in fearfulness, shyness and anxiety) will be obvious in a future post where the psychopath, not all but a vast majority, scores low in inhibitedness and therefore expresses what is known as boldness and fearless dominance, which, some argue, coupled with other traits, make the trait pathological in nature .
 Extreme Fear, Shyness, and Social Phobia (Series in Affective Science).
 Handbook of Psychopathy, Second Edition. Guilford Publications.
 Wikipedia contributors. “Essence.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 25 Feb. 2020. Web. 26 Apr. 2020.