One Man’s Modus Ponens is Another Man’s Modus Tollens

Sometimes you hear this expression “one man’s modus ponens is another man’s modus tollens” (hereafter MPMT) from philosophers and others. Here’s my understanding of it.

Modus Ponens and Modus Tollens are argument forms. Arguments are attempts to support a claim with reasons or evidence. The reasons/evidence are the premises, the claim being supported is the conclusion. Conclusions are often indicated by the keyword therefore.

Modus Ponens:
(1) If A, then B
(2) A
(3) Therefore, B

Example:
If today is Monday, then tomorrow is Tuesday.
Today is Monday.
Therefore, tomorrow is Tuesday.

Modus Tollens:
(1) If A, then B
(2) Not B
(3) Therefore, Not A

Example:
If today is Monday, then tomorrow is Tuesday.
Tomorrow is not Tuesday.
Therefore, today is not Monday.

Now for MPMT. It occurs when there are disagreeing parties. The first offers modus ponens. The opponent agrees with the first premise, but disagrees with the conclusion, and therefore, with the second premise.

It’s form:
Modus Ponens
(1) If A, then B
(2) A
(3) Therefore, B

Opposing Modus Tollens
(1) If A, then B
(2′) Not B
(3′) Therefore, Not A

The result is an battle between (2) and (2′). The first thinks that there is more behind A than Not B. The second thinks there is more behind Not B then A.

Here’s more of a real life example:

Conservative Modus Ponens:
(1) If freedom is more valuable than equality, then the government shouldn’t redistribute wealth.
(2) Freedom is more valuable than equality.
(3) Therefore, the government shouldn’t redistribute wealth.

Liberal Modus Tollens:
(1) If freedom is more valuable than equality, then the government shouldn’t redistribute wealth.
(2′) The government should redistribute wealth.
(3′) Therefore, freedom is not more valuable than equality.

These arguments agree on (1). The idea is that freedom and equality conflict at points. (Example, cis-gender women have an unequal share of dating opportunities. To make things equal, we’d have to force men to consider trans-gender women equally. To make everyone equal regarding income, we’d have to prevent some people from earning and keeping more.) They disagree on which of these two have the most behind it: That freedom is more valuable than equality, or that the government should redistribute wealth.

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8 thoughts on “One Man’s Modus Ponens is Another Man’s Modus Tollens

  1. “but that’s not objective authority”. And there is your issue. ‘Authored’ is not the same as ‘objective’; not every objective thing necessitates an author.
    I like this conversation, but to have it I think we need to agree a definition of “morality” before we move on. This is where the conversation needs to start, and it so rarely does. If you use the vague “what you ought to do” definition then you leave open all sorts of issues: the Nazis did what they ought to do according to the ideology enforced by Hitler; as did the Khmer Rouge. In fact, anyone that does anything is doing it because it is what they ‘ought’ to do according to the context the action is borne out of. “I ought to steal in order to fund my drug habit” is borne out of desperation, lack of money and selfishness.
    “Ought” necessitates a value system somewhere. And I think Sam Harris defines it in a way that captures the values people are referring to when they use the word: the safeguarding and promotion of net wellbeing.
    If we agree that is what morality is then morality is objective. And you don’t have to agree to the values inherent in this definition, but then others can call you “immoral” for your actions. Just like anyone who says 2+2=5 can be said to be non-mathematical (just because they don’t agree in the system that undermines arithmetic).

  2. All I have by way of reply my own intuition that “ought” isn’t relative to any sort of human thing. I like authority better than ought; I’m not thinking of authority as meaning “authored,” but that something (be it a person or an impersonal principle) has authority if it rightly demands submission from us. These principles might be simply written into the fabric of the universe as brute facts, or they might have an author. They are independent of human well being, though they often support it.

    I guess I’m thinking that it’s still an open question as to whether human beings ought to flourish. Maybe so. Maybe not, especially if it comes at the expense of something else that has greater weight.

    “And I think Sam Harris defines it in a way that captures the values people are referring to when they use the word: the safeguarding and promotion of net wellbeing.”

    I think that this is de facto and not de jure. People do think this way, but they also tend to think that the authority of morality is deeper than mere human agreement. A lot of people, for instance, think God’s authority is deeper than any human well being, but as God desires human flourishing (in the long term), it just happens to be that morality coincides with human flourishing.

  3. There are two types of authority: policed authority (i.e. going against the enforced rules leads to a punishment) and passive authority (we assume another should have authority over us).
    The first gives us no space for morality because it is derived the carrot and the stick, the latter gives us a morality as binding as the net-wellbeing-based morality Sam Harris describes.

  4. “There are two types of authority: policed authority (i.e. going against the enforced rules leads to a punishment) and passive authority (we assume another should have authority over us)”

    Maybe my intuitions are just different here. The authority I speak of is neither of these. The first isn’t really a moral authority at all, at best it is a prudential sort of wisdom (to avoid punishment).

    The second is all about our assumptions. It looks like both of these definitions are based on human behavior or something like that.

    My own view is that authority isn’t a naturalizable concept. It can’t be reduced to behavior. There is a primitive, basic, authority that certain principles have that we simply must (in a moral sense) follow.

    Can I prove this? No. At best I can merely point to intuitions about this absolute demand for obedience that certain imperatives make on us. So I admit that from an empirical point of view, my view and your number 2 are indistinguishable. But intuitively they seem different.

  5. I don’t agree with Sam Harris, because of his definition of objective morality. He defines objective good as “that which promotes the well-being of conscious creatures.” This isn’t really talking about objective moral values. He is trying to make objective moral values more about the flourishing of sentient beings, because Science can have a say in how that takes place. So, Voila! You have now a way of accessing objective moral values via the natural world, only because we have stripped the “value” from objective moral value.

  6. Russ Shafer-Landau goes about defending the idea of brute moral principles by dissecting the following inference:

    (1) Laws require lawmakers
    (2) Thus, if there are moral laws, then there must be moral lawmakers
    (3) These lawmakers are either us as we actually are, some idealized version of us, or God.

    Because Shafer-Landau is a moral realist, he rejects the idea that people are the lawmakers, and because he is a naturalist, he rejects the God hypothesis. To avoid the sting of the argument, he opts to reject (1) by reasoning as follows:

    If we assume that at least some laws are best construed realistically, then the truth of these standards (e.g. those of mathematics and chemistry) is not to be explained by their having been accepted or created by anyone. They just are correct. Now if mathematical or physical laws do not always require lawmakers, then perhaps moral laws do not require lawmakers. This means that either premise (1) is false, or global antirealism is true. And so he arrives safely back to the idea of eternal, necessary moral principles.

    But even if there are laws that are simply true, how does it follow that moral ones are, or
    could be, that way? The theist could also reject (1) and argue that there are only certain kinds of laws that require lawmakers, with moral laws falling into that category. So, by modus ponens:

    (2) If there are moral laws, then there must be moral lawmakers.
    (2*) There are moral laws.
    (3) These lawmakers are either us as we actually are, some idealized version of us, or God.

    On Moral Realism, then,

    (4) The moral lawmaker is God

    follows.

    The naturalist may reject (2) on the grounds that it is not obviously true, perhaps reverting back to Shafer-Landau’s argument from mathematical laws or laws of physics or chemistry. But this amounts to mixing apples and oranges. Moral laws are nothing like mathematical laws. Moral laws are normative or prescriptive and the laws of math and science are descriptive. The former tells people how they ought to live, while the latter tells them about how the world is and how it works. Of course, there are normative elements to laws of math and logic since they show people how to think correctly about the world. But a mistake in reasoning is not at all the same as a moral blunder. A person is not blameworthy for erring in math or science unless those errors were committed on purpose, either out of malice or deceit, or out of an apathy or laziness. But the malice, the deceit, the apathy and laziness are moral failings. They have nothing to do with the math or science.

    To emphasize the degree of separation, different terminology might be employed to describe the two realms. Instead of speaking of moral “laws,” it might be preferable to speak of moral “decrees” or moral “mandates.” It is much easier to capture the prescriptive force of morality when the language takes this kind of form. Unfortunately for the naturalist who wants to be a moral realist, using these terms makes it much easier to see how the prescriptive force of morality derives from an objective, authoritative, intentional agency.

  7. Thank you for the thoughtful comment. I agree with most of you say, though I don’t think (3) follows from (2) and (2*), for all they imply is that some sort of person is responsible for the moral laws, not necessarily us or God. But I do think it raises the probability of God’s existence if we exclude ourselves from the list, as we don’t readily think of other sorts of beings capable of this. The point about math and science being descriptive, versus morality being prescriptive is well-taken, but would this necessarily point to a person being behind them?

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