If only

Part of a series which began with Fake news and the ethics of belief.

Last time we looked at how evidence can support a descriptive belief. We will now see if evidence can also support a prescriptive belief.

Take the first on our list:

I ought to do more exercise

Is this a moral imperative, like ‘I must not kill’?

That doesn’t seem right. It is more a ‘pragmatic’ imperative, like ‘I ought to use a Phillips screwdriver on a cross-head screw’.

Phillips screwdriver

At first sight it does look as if a pragmatic prescriptive belief like ‘I ought to do more exercise’ can be supported by evidence. After all we can compare people who exercise with people who don’t, and see which are more healthy. We can also look at scientific studies on the effect of exercise on people.

But these all presuppose that there’s something we’re trying to achieve: to get healthy or stay healthy. The imperative to do more exercise only makes sense in the context of an intention (or desire or objective) which obeying the imperative is likely to achieve.

Imagine a reality TV star who is being sponsored by a manufacturer of gym wear. Her contract stipulates a minimum percentage of screen time where she has to be seen exercising, otherwise she will start to lose income.

In this case the evidence supporting her belief that she ought to do more exercise might be records of screen time and/or emails from her sponsor complaining she might be in breach of contract.

So it looks as if pragmatic prescriptive beliefs can only be supported by evidence in the light of whatever the intention is. If we ignore the intention then we cannot specify what sort of evidence will be relevant, because the evidence might be different for different intentions.

yoga exercise

If we do spell out the intention, then we end up with hypothetical prescriptive beliefs. For example:

[IF I want to improve my health THEN] I ought to do more exercise

[IF I want to protect my sponsorship income THEN] I ought to do more exercise

But for each of these (hypothetical) prescriptive beliefs we could come up with an equivalent descriptive belief which preserves its sense:

The more exercise I do, the more my health improves

Doing more exercise will help me protect my sponsorship income

These are both straightforward descriptive beliefs which can be supported by evidence – the same kinds of evidence which can support the equivalent hypothetical prescriptive beliefs.

Now compare this with a moral imperative like ‘I must not kill’. This is categorical, not hypothetical. It is not incomplete or ambiguous in the absence of a presupposed intention. If we do specify an intention, then it stops being a moral imperative:

[IF I want to stay out of prison THEN] I must not kill

This is not a moral imperative but a pragmatic imperative concerned with preserving one’s freedom. We could replace this with an equivalent descriptive belief, which we could then support with evidence. Something like:

Refraining from killing other people will help me stay out of prison


Killing other people will lead to me being put in prison

But this doesn’t tell us anything about whether evidence will support that original categorical, moral, prescriptive belief.

Read on.

© Chris Lawrence 2021

3 thoughts on “If only

  1. Pingback: Evidence, m’lud | some strong language

  2. Pingback: Taken for a mug | some strong language

  3. Pingback: *Press This* If only #168 | Its good to be crazy Sometimes

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