1

Source: p 115, Philosophy: A Complete Introduction (2012) by Prof Sharon Kaye (MA PhD in Philosophy, U. Toronto). Caution: I rewrote numerals as integers for easier reading.

1 Relations of ideas are logical truths, such as '2 + 2 = 4'. These are necessary because the attempt to deny them results in a contradiction.
If 2 + 2 = 5, then 1 + 1 + 1 + 1 is not 1 + 1 + 1 + 1, which is a contradiction.

2 Matters of fact are observed truths, such as 'bread nourishes'. These are contingent (meaning 'not necessary') because you can deny them without producing a contradiction. [3.] The idea of bread poisoning instead of nourishing involves no conceptual impossibility. [4.] We can prove this by imagining a world where bread poisons instead of nourishes.

Hume points out that the assertion 'A causes B' is a matter of fact, not a relation of ideas. Therefore it can't be necessary.

Of course, everyone recognizes instances when matters of fact go differently from usual. We've all eaten something that usually nourishes us, only to find that, this time, it made us sick. When this happens, we are liable to look for a hidden cause — some microscopic bacteria in the food that gave us food poisoning.

But when Hume calls the nourishing effect of bread a matter of fact, he isn't saying that there may be hidden causes that make things turn out differently this time. [5.] He's saying that the bread could suddenly have a completely different effect without any different causes at all.

To insist that there would have to be different cause in order to produce a different effect would be to cast the nourishing effect of bread as a relation of ideas like '2 + 2 = 4'. Because human beings are creatures of habit, we constantly view causal connections as logical connections. But this is a conceptual confusion we must overcome, in Hume's view.

What abstract distinctions between 1 and 2 have I neglected? I do not understand

3 and 4: because one necessary condition for 'bread nourishes' is: the bread is not poisoned (with bacteria). They are mutually exclusive.

5: how can no change in any cause still cause completely different effects? With no change in any cause, how can nourishing bread suddenly mutate into poisoned bread?

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    He means that we can imagine an "alien creature" with a different biology and physiology such that if it eat bread it will die: this "imaginary fact" is not logically impossible (it is quite real: see Coeliac disease). – Mauro ALLEGRANZA Mar 7 '16 at 20:07
  • @Mauro ALLEGRANZA +1. Thank you for the example! I empathise with such patients. – Greek - Area 51 Proposal Mar 18 '16 at 4:35
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The statement

This thing is a nourishing bread

can be expanded into:

This thing was a nourishing bread in the recent past, and will be a nourishing bread in the near future

Hume argued, however, that such a statement would be an invalid way to describe what we know and experience. One reason is that, according to Hume, we cannot justifiably assert anything about the future. So an improvement would be:

This thing was a nourishing bread in the recent past (and that's it)

And nothing is rationally implied about what will happen in the future. In particular, the possibility that the past nourishing bread will turn into a poisonous one cannot be ruled out.

A consideration of causes cannot be used to argue against Hume, because Hume's thesis includes a related criticism against the very idea of cause. The most we can tell about causes, according to Hume, is that events of one type were consistently followed by events of another type - until now. Again we cannot, according to Hume, rationally assert anything about causal relations continuing to hold in the future. So nothing about causes, as well, would preclude the possibility that a nourishing bread will turn just like that into a poisonous one.

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