At first sight, the word 'intrinsic' appears to be a virtual - or even literal - synonym of the word 'essential'. Indeed it's tempting to use the latter - rather than former - word. However, as often happens in philosophy, there are indeed slight differences between the two ontological categories. Or, at the very least, there are different definitions of the words 'intrinsic property' and 'essential property'. Nonetheless, it can still be said that the two categories are very closely related. Or, to put that another way, this “sign-substitution” (to use Derrida's term) of 'intrinsic' for 'essential' would never have happened if essentialism and anti-essentialism had never been such important parts of the Western philosophical tradition.
In any case, some metaphysicians tell us that there's a difference between properties which objects have independently of any external factors acting upon them (i.e., intrinsic properties) and properties which are deemed to be the way they regardless of what's external to them (i.e., essential properties). Despite saying that, can't that account of intrinsic properties also be applied to essential properties? Can't we also say that essential properties are those properties which are independent of any - or all - external factors?
It's true that this wasn't how essential properties were usually defined in the tradition. Nonetheless, it actually seems like a good definition. And if that's the case, then what's happened here isn't the discovery of another ontological category: it's a new way of accounting for an old ontological category. That is, when essential properties are defined in such a way as to emphasise their independence from all external factors, then those properties become intrinsic in nature – even though they're exactly the same as essential properties! Again, the only thing that's changed are the definitions and/or accounts.
David Lewis on Intrinsic Properties
This is David Lewis's definition of intrinsic properties :
“A thing has its intrinsic properties in virtue of the way that thing itself, and nothing else, is.”
Could there ever be such a state as “the way that a thing itself is” regardless of everything else? That is, regardless of its relations to other properties/objects/events; its place in time and space; and so on?
This position is taken to its most extreme (or perhaps ridiculous) in the following statement:
Object a would still have intrinsic property P if after the world around it disappeared, a would still have P.
(This is a little like Max Black's two-sphere universe; as seen in his paper 'The Identity of Indiscernibles', 1952.)
Can't we still say that there are intrinsic properties; though they still have vital relations to extrinsic properties? That is, extrinsic proprieties may determine - to some extent at least - intrinsic properties. However, it may be countered that because objects are such-and-such-a-way, then they can only be affected or determined in particular ways because they have intrinsic properties which are the way they are. That means that there may be some kind of mutual relation between intrinsic and extrinsic properties; as well as between extrinsic and intrinsic properties. Indeed, as I said, there may be no “way” an object is regardless of its relations to other things or to extrinsic properties.
David Lewis also cited “internal structure” as being intrinsic to objects. Yet if structure is defined in terms of its relations, then surely it must also be defined (partly) in terms of extrinsic properties. Thus Lewis's internal structures would be defined - or even constituted - by external relations or extrinsic properties. It can now be asked what would be the point of a Lewisian internal structure if it weren't primarily a crutch (or framework) for intrinsic properties which bore no such relations to external factors.
It can now be said that internal structures determine relations and therefore also determine extrinsic properties. Then again, it can equally be said that external relations (or extrinsic properties) determine internal structures (or intrinsic properties). Here again the boundaries between what's intrinsic and what's extrinsic seems to somewhat blur.
David Lewis also wrote something about an intrinsic property which isn't entirely helpful until one's entirely sure what such a property actually is. He wrote:
“ … If something has an intrinsic property, then so does any perfect duplicate of that thing; whereas duplicates situated in different surroundings will differ in their extrinsic properties.” 
The problem with that definition (or stipulation!) is that if a “thing” is in “different surroundings” it may also have different intrinsic properties to the ones it had in its previous surrounding/s. Or to put that more clearly:
i) Object a has set of intrinsic properties I in surrounding S.
ii) Object a has set on intrinsic properties I2 in surrounding S2.
To put that in more basic words, object a may change its intrinsic properties (not only its extrinsic properties) in different places. And that, surely, will depend on its relations to other objects; as well its relations to events or properties.
Surely the conclusion to this is that it's very hard to distinguish intrinsic from extrinsic properties. Thus why not give up on the distinction altogether?
In argument form:
i) If object a changes its intrinsic properties in “different surroundings” (Lewis),
ii) then a distinction between intrinsic & extrinsic properties will be difficult to make.
iii) Therefore get rid of the distinction between intrinsic & extrinsic properties entirely.
There's an additional way of looking at conclusion iii) above. Thus:
i) If the intrinsic/extrinsic distinction fails for objects,
ii) and the having of intrinsic properties is said to be fundamental for the discernibility, individuation, etc. of objects,
iii) then the ontological reality of objects itself may be questioned.
Shape as an Intrinsic Property
Lewis believed that the shapes of (some?) objects are intrinsic to such objects. However, don't the shapes of many – or all – objects change through time? Indeed one shape-changer is said to be the curvature of space itself. More correctly, objects (at least to some minute degree in many cases ) help curve space and space itself helps shape objects. Thus shape will depend on the curvature of space. What's more, the curvature of space may be continuously working as a shape-changer.
Unless, that is, object a's shape at time t is intrinsic and object a's (slightly different) shape at t2 is then also intrinsic to it. Though if a's shape were always changing, its intrinsic properties would also be changing. That's because it would have two intrinsic shapes at two different times. And that seems to go against the notion of intrinsicality.
Having just put the case for space's impact on the shape of objects, I'm not entirely convinced by it.
My first thought was that the curvature of space could never have an impact on the shape of (as J.L. Austin put it) “medium-sized dry goods” (or macro-objects). The curvature of space does indeed have an impact on how such objects travel through space (as well as vice versa); though even here the impact is minute. (Massive objects, such as the earth, are a different matter.)
So what about particles and other micro-phenomena? (This ties in with the lack of a theory of "quantum gravity".) I believed that the curvature of space didn't have any impact at the quantum level; though I may be wrong about that. After all, such curvature only occurs at the extremely-large scale. And, as I've just said, it won't even have an impact on everyday objects such as persons or trees either.
An interesting addition to the theory of intrinsic/external properties would be the category of intrinsic relations.
Intrinsic relations are said to determine or even constitute objects. In other words, they're fundamental to the objects which have them.
That's primarily the case because an object's intrinsic relations to other objects (or set/s of properties) are actually constitutive of what that object is. In other cases, an object's relations aren't constitutive – or part - of that object's fundamental nature.
For example, the property [being two mile away from] can be deemed to be an intrinsic relation. Similarly, the property [being the same species as] can also be seen that way.
Clearly, if object a is two miles away from object b, then b must also be two miles away from a. Thus, in this case, the property [two miles away from] is symmetrical in regards to a and b.
Similarly with the property [being the same species as]. If object a is the same species as object b, then b must be the same species as a.
However, how can relational properties be deemed to be intrinsic? It's certainly counterintuitive to say that being two miles away from could be an intrinsic property or, indeed, to be any kind of property of an object... Unless, of course, that relation remains constant through time.
What about the property [being the same species as]? Surely in the case of object a we can say that its being a member of species S is fundamental or intrinsic. However, why should we say the same about the property [being the same species as]? That seems to be a needless addition to the ontology of properties.
It can certainly be said that a pair of particles can be said to display or partake of intrinsic relations.
For example, say that electron a and positron b stand in relation R to one another. (Many other complex aspects of quantum entanglement may make the following statements problematic; if not outright false.)
R is an intrinsic relation iff a (always) stands in R to b and b (always) stands in R to a.
That relation will also determine the nature of both a and b. In fact it can be said that the intrinsic relation is actually constitutive of the natures of both a and b.
Electron a and positron b (or any pair of objects), on the other hand, may be related to all sorts of other objects, properties or events which don't help constitute their fundamental nature or intrinsic nature/properties.
In ontic structural realism, this category of intrinsic relations is certainly taken to be true about subatomic particles. Yet ironically this leads (some?) ontic structural realists to deny that there are objects at all.
Could this also be applied to macro-objects such as persons or trees? And need we follow the ontic structural realists in denying that there are objects simply because they have fundamental relations to - or indeed are partly constituted by – other objects, events or extrinsic properties?
If there were a “perfect duplicate” of David Cameron, according to Lewis, then that duplicate and Cameron would share the same intrinsic properties.
Thus the idea of a perfect duplicate sharing all intrinsic properties is helpful for a possible-worlds theory in which counterparts share such intrinsic properties. Thus:
If a and b are of the same kind (or are duplicates), then they must share all their intrinsic properties (even if they have different extrinsic properties).
In a certain sense, it's said that objects and even persons must have intrinsic properties in order to exist as the objects and persons that they are over time. If there were no intrinsic properties, then an object or person would only last for a second or even less. Thus could it really be seen as an object or self/person in the first place?
However, if there were a duplicate of Cameron, wouldn't that duplicate also share all his extrinsic properties? To put that more simply: wouldn't it share all his properties by virtue of it/him being a duplicate? This conclusion may simply amount to the inappropriate usage of the word 'duplicate' on Lewis's part. After all, on Lewis's picture, counterparts only duplicated the intrinsic/essential properties of the objects they are counterparts of.
But wouldn't that be to beg the question?
Counterpart theory is used to distinguish intrinsic from extrinsic properties. However, in order to have duplicates one must already be committed to intrinsic properties in order to define what is and what isn't duplicated. So rather than discovering intrinsic properties through counterpart/duplicate theory, one actually assumes them. Moreover, not only is the existence or reality of intrinsic properties assumed, so is what is and what isn't an intrinsic property.
Lewis, David, 'Extrinsic Properties' (1982), Philosophical Studies (44: 197–200).
- On the Plurality of Worlds (1986a), Oxford: Blackwell.
- 'Rearrangement of Particles: Reply to Lowe' (1988), Analysis (48: 65–72).