Thursday 17 September 2020

On the Need to Distinguish Knowledge of Essential Properties from the Knowledge That They Are Essential

In one of my favourite papers about modal metaphysics, Rosen's 'The Limits of Contingency', Rosen develops the idea that there are two sets of modal notions which are apt to be conflated under the heading 'metaphysical modality'. Roughly: a correct conceivability modality, and a more properly metaphysical modality on which some things which are correctly conceivable are in any case not really possible.

In a recent paper, 'Correct Conceivability and its Role in the Epistemology of Modality' whichin a new book Les Principes Métaphysiques published by the College de France, Robert Michels considers an objection to Rosen's idea of a correct conceivability modality construed as giving an account of how considerrations about conceivability can takes us from non-modal to modal knowledge. I think this objection is based on a misunderstanding, as I will explain in this post. 

Let me quote from Michels, first to set the scene, and then to see the objection and the main responses Michels considers.

Correct Conceivability

The basic idea

10The correct conceivability-approach presupposes the Kripkean standard view of metaphysical modality and accordingly has to rule out modal errors, cases in which a state of affairs is conceivable, but not metaphysically possible. How this is done is nicely explained in the following mock-quote of Rosen’s “others”:

If the ancients could conceive a world in which water is an element, this is only because they were ignorant of certain facts about the natures of things. In particular, it is because they did not know what it is to be water. They did not know that to be water just is to be a certain compound of hydrogen and oxygen – that to be a sample of water just is to be a quantity of matter predominantly composed of molecules of H2O. This is not to say that they did not understand their word for water. But it’s one thing to understand a word, another to know the nature of its referent. The ancients could see no contradiction in the supposition that water is an element because they did not know that water is a compound by its very nature. But we know this; and given that we do, we can see that to suppose a world in which water is an element is to suppose a world in which a substance that is by nature a compound is not a compound. And that’s absurd. (Rosen 2006, p. 22-23)

11The idea is hence that in order to avoid modal error, conceivability needs to be supplemented by knowledge of the natures, or equivalently, essences of relevant entities, in this case the essence of (the property of being) water. Equipped with this knowledge, the conceiver is able to detect that the assumption that water is an element together with essential truths about the relevant entity entails an absurdity.

12The correct conceivability-approach hence gives us a simple and elegant explanation of why we are apt to make modal errors and a recipe for ruling them out. We tend to commit modal errors because we can conceive of states of affairs which are ruled out by relevant essences. To avoid these errors, we have to let our ability to conceive be guided by knowledge of the essences of relevant entities.

An objection to essence-based conceivability approaches and how it can be addressed

13There is a rather obvious objection to the correct conceivability-approach which should be mentioned: The approach crucially relies the conceiver having knowledge of essence, but essence itself is a modal notion. Doesn’t this mean that the notion of correct conceivability cannot answer the core question of modal epistemology, the question of how we can acquire modal knowledge?

  • 5 See for example Lowe (2012), Hale (2013, ch. 11), Tahko (2016), Tahko (2017). For a recent critical (...)


(End of quote.) 

Michels considers a number of responses, but not the following simple one which I think is correct.

Yes, the approach relies on the conceiver knowing that some thing has a property, where arguably that property is in fact an essential property of the thing. But the approach does not rely on the conceiver knowing that the thing essentially has that property.

The idea is not that people learned that water is essentially H2O and then on that basis, saw that water being simply an element is not correctly conceivable. Rather, they just learned that water is H2O, and then on the basis of their grasp of how the concept of water works, saw that water being an element is not correctly conceivable. The knowledge of essence, just like the knowledge of necessity (Fine taught us to disinguish the two), may perhaps also be accounted for along similar lines. But that's by the by. This objection simply doesn't get off the ground as far as I can see. 

This sort of confusion is not unique to Michels's paper. Phrases like 'knowledge of essence' and 'knowledge of necessity' are apt to blur this important sort of distinction, leading to mistakes in the epistemology of modality. 


Michels, Robert. Correct Conceivability and its Role in the Epistemology of Modality In : Les principes métaphysiques [en ligne]. Paris : Collège de France, 2020 (généré le 18 septembre 2020). Disponible sur Internet : <>. ISBN : 9782722605350. DOI :

Rosen, Gideon (2006). The limits of contingency. In Fraser MacBride (ed.), Identity and Modality. Oxford University Press. pp. 13--39.