Thursday, November 20, 2008

Duties of conscience

Suppose that one affirms:

  1. One ought always to do what conscience requires.
  2. Conscience is fallible.
Then, I think, one ought also affirm:
  1. It is possible for one to be obliged to do A and to be obliged to do B, where doing both A and B is logically impossible.
  2. It is possible for one to be obliged to do something logically impossible.
The argument is plausibilistic. A conscience that can err is not going to be infallible about detecting logical incompatibilities and impossibilities. As a result, it should be possible for it to erroneously recommend something impossible, or to recommend two incompossible actions.

We can even give examples. Suppose I have a justified belief that I promised to draw a triangle, and another justified belief that I promised never to draw a figure whose angles add up to 180 degrees. A fallible conscience need not detect the incompatibility between the content of these two promises. And if it does not, then conscience will require me to do both things, even though it is impossible. Similarly, if I do not know that it is impossible to construct a 20 degree angle with straightedge and compass, and I have promised to construct a 20 degree angle with straightedge and compass, then conscience will require me to construct a 20 degree angle with straightedge and compass. But it is logically impossible (assuming that part of the concept of "construct" here is an assumption of the axioms of Euclidean geometry) to construct a 20 degree angle with straightedge and compass.[note 1]

Some may take this to be a strong argument against the conjunction of (1) and (2). Others may take this to be just an interesting consequence of it. I don't myself know what to do with this.


Sardonicus said...

Prof. P.,

The realization that one is obliged to do a logically impossible task would seem to:

1. Discharge the obligation or;

2. Require one to modify the obligation in some way so as to meet the original purpose of undertaking the obligation.

Now, it seems to me that there are different kinds of fallibility. One sort is empirical fallibility - thus, one could commit to draw a triangle with total angles of 145 degrees when one lacks knowledge that such a thing is impossible. When one gains such knowledge, the obligation must be discharged or modified. Another sort is a moral fallibility, where the conscience is mistaken about the nature of some act.

There may be others here.


Alexander R Pruss said...

But what if one never realizes it's impossible? Then, it seems, one ought to do it, assuming one ought to do what conscience says.

We can even give cases where it might be impossible (maybe not logically, though) for us to know that the thing is impossible. Suppose that T is a mathematical proposition, which unbeknownst to us is a Goedelian unprovable mathematical truth T. We could promise to produce a counterexample to T. And we could never know (apart from something like divine revelation) that this impossible.

Brandon said...

I think a third way to argue is that the following inference is invalid for obligations:

(1) Oa & Ob
(2) Therefore, O(a&b)

That is, even if you are obliged to do a and obliged to do b, it doesn't necessarily follow that you are obliged to do a and b both (even if it usually is true). (It's like wanting: If I want an ice cream cake and I want a german chocolate cake, it doesn't follow that I want both an ice cream cake and a german chocolate cake, because I may want only one cake. Wi & Wg doesn't yield W(i&g).)

I'm not sure that I would take this path myself.

Alexander R Pruss said...


In my arguments, as far as I can tell I never made use of the inference you criticize. Maybe your point is that (3) is only counterintuitive if one accepts that inference. But I think (4) will be counterintuitive even if one does not accept that inference.

Brandon said...

As far as I can see, you can't get from (3) to (4) without such an inference. (3) is:

{Poss}({Obl}A & {Obl}B)

To get (4) you have to take this to imply


Which, given the assumption that doing both A&B is logically impossible, gives you (4). This is true even if we take the inference only to be plausible. If you don't make this inference, you simply have the claim that you have two obligations, one for each conjunct of a conjunction that is impossible, not that you are obliged to do what is described the impossible conjunction. What am I missing in your argument, if it doesn't use this inference?

Tom said...

One byway: a well-formed conscience will prevent you from promising to do something you don't know you can do.

Heath White said...

Jonathan Dancy has an article on this very issue ("The Logical Conscience"). His solution is that the first obligation is not

If your conscience says do X, you ought to do X


You ought(if your conscience says do X, to do X).

Which I think is equivalent to

You ought either reform your conscience or do X.

That effectively solves the puzzle, though it may not be the best gloss on the duty to follow conscience. (It is not the *worst* gloss on that duty, though.)

Alexander R Pruss said...


Maybe. But a well-formed conscience is no protection against false memories. You might have a false memory of having promised to construct a 20 degree angle!


What if reforming your conscience is impossible (e.g., for psychological reasons)? Then, I think, we get the same view.

Moreover, I think that oughts are best considered in the context of deliberation between open options. We can suppose a case where you must act right away. There is no time for reforming your conscience.