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\renewcommand{\P}{\mathrm{P}}
\renewcommand{\|}{\:\vert\:\mathopen{}}
\newcommand{\mo}{\mathrel{\!=\!}}

# Introduction

The probability calculus is an extension of the calculus of propositional logic. Besides the *truth* or *falsity* of propositions, the probability calculus also considers a continuum of degrees of credibility -- probabilities -- between those two extremes. Just as propositional logic gives a set of rules to derive the truth or falsity of some propositions from the truth or falsity of others, so the probability calculus gives a set of rules to derive the probability for some propositions from the probability for others; these rules include the logical ones as a special case.

There are different logical deduction systems (Pelletier & Hazen 2023), each with many dialects, to express the rules of propositional logic. One of them is the [*sequent calculus*](https://encyclopediaofmath.org/wiki/Sequent_calculus) (Takeuti 1987). The probability calculus has many analogies with the sequent calculus

In sequent calculus we express that proposition $a$ is true within a set of axioms $I$ by the notation

$$
I \vdash a
$$

The same is expressed in the probability calculus by the notation (note the reflection)

$$
\P(a \| I) = 1
$$

but in this case we can also consider degrees of credibility different from $1$.

The theory of "random variables" is a particular application of the probability calculus: an expression such as "$X\mo x$" means "The quantity $X$ is measured to have value $x$" -- which is just a proposition. Also the "causal calculus" is a particular application: an expression such as "$\mathrm{do}(X \mo x)$" means "The quantity $X$ has been *set* equal to $x$" -- which is also just a proposition. In the probability calculus we can in fact even consider more general propositions, such as "The quantity $X$ has been *reported* to be equal to $x$".

\

The view of the probability calculus as an extension of the logical calculus goes back at least to Boole, possibly to Laplace. For more information about the formalism and history see e.g. Hailperin 1996, Jaynes 2003, Jeffreys 1983, Johnson 1924, Johnson 1936.


# Probabilistic inferences

In propositional logic, suppose we assert that:

- proposition $a$ is true given a set of axioms $I$,
- proposition $b \lor \lnot a$ is true given the set of axioms $I$ augmented with the proposition $c$.

Then we can also assert that $b$ is true given the set of axioms $c \land I$. This is a logical inference. In the notation of sequent calculus (Takeuti 1987) it is written as follows:

$$
\frac{
I \vdash a \quad\quad c \land I \vdash b \lor \lnot a
}{
c \land I \vdash b
}
$$

The conclusion follows from the initial assertions by the application of a set of inference rules.

In the probability calculus the same inference can be expressed as follows:

$$
\frac{
\P(a \| I) = 1\quad\quad \P(b \lor \lnot a \| c \land I) = 1
}{
\P(b \| c \land I) = 1
}
$$

This final probability can be shown to follow from the initial ones by the well-known probability rules, valid for any sentences $a$, $b$:

- $\P(\lnot a \| I) = 1 - \P(a \| I)$

- $\P(a \land b \| I) = \P(a \| b \land I)\cdot\P(b \| I)
= \P(b \| a \land I)\cdot\P(a \| I)$

- $\P(a \lor b \| I) = \P(a \| I) + \P(b \| I) - \P(a \land b \| I)$

- $\P(a \| a \land I) = 1$


Another example of a simple probability inference, which immediately follows by the probability rules, is

$$
\frac{
\P(a \| I) = 0.3\quad\quad \P(b \| a \land I) = 0.2
}{
\P(a \land b \| I) = 0.06
}
$$

\

The probability rules effectively imply the rules of propositional logic as special cases.

What's remarkable is that **the probability rules allow us to algorithmically determine the *lower* and *upper* values that a probability can have, under the assertion of the values of other probabilities**. It is well-known, for instance, that if

$$
\P(a \| I) = 0.2\,,\quad
\P(b \| I) = 0.7
$$

then the probability $\P(a \land b \| I)$ cannot be larger than the minimum of the two above, that is,

$$
\P(a \land b \| I) \in [0, 0.2]
$$

In fact this is also possible with inequality constraints. As a very trivial example, if the probability $\P(a \| I) \le 0.3$, then we must have $\P(\lnot a \| I) \ge 0.3$.

Finding such bounds is equivalent to solving a linear-optimization problem. The most relevant texts about this equivalence are those by Hailperin 1965 and Hailperin 1996.

# The function `inferP()`

The function `inferP()` finds the bounds of a target probability by implementing the linear-optimization algorithm mentioned in the previous section.

As a first argument,`target =`, it takes the conditional probability of two   logical expressions; as optional subsequent arguments it takes equality or inequality constraints about the conditional probabilities of other logical expressions.

The main notation used in the function (corresponding to the default argument `solidus = TRUE`) parallels the mathematical notation used above, with the following differences:

- Not: ` !` or ` -`
- And: ` & ` or ` && ` or ` * `
- Or: ` + `
- If-then: ` > ` ("if $a$ then $b$", `a > b`, is simply equivalent to $b \lor \lnot a$)
\

If an additional argument `solidus = FALSE` is given in `inferP()`, then a notation is used that differs from the above in the conditional symbol and in the "or"-connective:

- Conditional solidus "${}\|{}$": ` ~ `
- Or: ` + ` or ` | ` or ` || `

This alternative notation allows the user to use proper R syntax for the connectives (`!`, `&`, `|`, `&&`, `||`), in case it were convenient to paste them from other R expressions.

In the following examples the main notation is used.


## Simple examples

Load the package:

``` r
library('Pinference')
```

Here are some examples, from more trivial to more complex.

- The probability for a sentence $a$ can a priori be anything between 0 and 1:

``` r
inferP(
    target = P(a  |  I)
)
# min max 
#   0   1
```

\

- But if we assume the truth of that sentence, then its probability must be 1:

``` r
inferP(
    target = P(a  |  a & I)
)
# min max 
#   1   1
```

\

- If we assume the negation of that sentence, then its probability must be 0:

``` r
inferP(
    target = P(a  |  !a & I)
)
# min max 
#   0   0
```

\

- A probability conditional on contradictory premises, such as $b \land \lnot b$, is undefined:

``` r
inferP(
    target = P(a  |  b & !b & I)
)
# min max 
#  NA  NA
```
\

## Examples with constraints

- This is a statement of the "and" or "multiplication" or "conjunction" rule:

``` r
inferP(
    target = P(a & b  |  I),
    P(a  |  I) == 0.2,
    P(b  |  a & I) == 0.3
)
#  min  max 
# 0.06 0.06
```

- The rule for "conditional probability":

``` r
inferP(
    target = P(b  |  a & I),
    P(a & b  |  I) == 0.06,
    P(a  |  I) == 0.2
)
# min max 
# 0.3 0.3
```

- Same rule, but with an inequality constraint:

``` r
inferP(
    target = P(b  |  a & I),
    P(a & b  |  I) == 0.06,
    P(a  |  I) <= 0.2
)
# min max 
# 0.3 1.0
```

- If we assert that two sentences have the same probability, and the sentences are mutually exclusive and exhaustive, then each must have 0.5 probability:

``` r
inferP(
    target = P(a  |  I),
    P(a  |  I) == P(b  |  I),
    P(a & b  |  I) == 0,
    P(a + b  |  I) == 1
)
# min max 
# 0.5 0.5
```

- The logical rule of implication (*modus ponens*): if $a$ and $a \Rightarrow b$ are true, then $b$ is true:

``` r
inferP(
    target = P(b  |  I),
    P(a  |  I) == 1,
    P(a > b  |  I) == 1
)
# min max 
#   1   1
```

- Under a false proposition, any implication is always true:

``` r
inferP(
    target = P(a > b  |  I),
    P(a  |  I) == 0
)
# min max 
#   1   1
```

- A variant of the [cut rule](https://ncatlab.org/nlab/show/cut+rule/) of sequent calculus:

``` r
inferP(
    target = P(X + Y | I & J),
    P(A & X | I) == 1,
    P(Y | A & J) == 1
)
# min max 
#   1   1
```
\

## Combining evidence

Suppose that hypothesis $H$ has probability 0.7 given evidence $E_1$, and probability 0.8 given evidence $E_2$. What is its probability if two pieces of evidence are combined, that is, given their conjunction? An interesting result, proven in Hailperin 2006, is that such probability can be anything:

``` r
inferP(
    target = P(H  |  E1 & E2 & I),
    P(H  |  E1 & I) == 0.7,
    P(H  |  E2 & I) == 0.8
)
# min max 
#   0   1
```

# The Monty Hall problem

The "Monty Hall problem", inspired by the TV show *Let's make
  a deal!* hosted by Monty Hall, was proposed in the *Parade* magazine
in 1990 (see Lo Bello 1991). the numbers of the doors are changed here):

> Suppose you are on a game show and given a choice of three doors. Behind one is a car; behind the others are goats. You pick door No. 1, and the host, who knows what is behind them and wouldn't open the door with the car, opens No. 3, which has a goat. He then asks if you want to pick No. 2. Should you switch?

We can solve this problem with `inferP()`.

## Propositions

First let's introduce some notation for the relevant propositions:

- `I`: a proposition expressing the rules of the game,
- `car1`: "The car is behind door No. 1",
- `you1`: "You pick door No. 1",
- `host1`: "The host opens door No. 1",

and similarly for the other doors.

## Target probability

We want to know what's the probability that the car is behind the other door, No. 2.

The conditional of the target probability has, besides the game rules $I$, three propositions expressing the information we have: you chose door No. 1, `you1`, the host opened door No. 3, `host3`, and there is no car behind door No. 3, `!car3`.

The target probability is thus `P(car2  |  you1 & host3 & !car3 & I)`

## Given probabilities

We must express the information we have and the game rules in terms of probabilities. Note that there are several equivalent ways to express the probabilistic constraints listed below.

1. We know that there is only one car, and it must be behind one door. This corresponds to four probability constraints:    
    `P(car1 & car2  |  I) == 0`    
    `P(car1 & car3  |  I) == 0`    
    `P(car2 & car3  |  I) == 0`    
    `P(car1 + car2 + car3  |  I) == 1`
\

2. The host must open one door, and only one door. But the host cannot open the door you chose, and cannot open the door that has the car, but must open one door. This corresponds to seven probability constraints:    
    `P(host1 & host2  |  I) == 0`    
    `P(host1 & host3 |  I) == 0`    
    `P(host2 & host3  |  I) == 0`    
    `P(host1 + host2 + host3  |  I) == 1`    
    `P(host1  |  you1 & I) == 0`    
    `P(host2  |  car2 & I) == 0`    
    `P(host3  |  car3 & I) == 0`    
\

3. Let's say that you are initially equally undecided among the three doors:    
    `P(car1  |  I) == P(car2  |  I)`    
    `P(car2  |  I) == P(car3  |  I)`    
\

4. The fact, alone, that you picked door No. 1 is irrelevant to the knowledge about the car's position:    
    `P(car1  |  you1 & I) == P(car2  |  you1 & I)`    
    `P(car2  |  you1 & I) == P(car3  |  you1 & I)`    
\

4. Finally, we have no reason to believe that the host favours one door over another, when a choice is possible, that is, when the car is behind the door you chose:    
    `P(host2  |  you1 & car1 & I) == P(host3  |  you1 & car1 & I)`
\

## Result

We can finally input the desired target and the constraints into `inferP()`:


``` r
inferP(
    target = P(car2  |  you1 & host3 & !car3 & I),
    ##
    P(car1 & car2  |  I) == 0,
    P(car1 & car3  |  I) == 0,
    P(car2 & car3  |  I) == 0,
    P(car1 + car2 + car3  |  I) == 1,
    ##
    P(host1 & host2 | I) == 0,
    P(host1 & host3 | I) == 0,
    P(host2 & host3 | I) == 0,
    P(host1 + host2 + host3  |  I) == 1,
    P(host1  |  you1 & I) == 0,
    P(host2  |  car2 & I) == 0,
    P(host3  |  car3 & I) == 0,
    ##
    P(car1  |  I) == P(car2  |  I),
    P(car2  |  I) == P(car3  |  I),
    ##
    P(car1  |  you1 & I) == P(car2  |  you1 & I),
    P(car2  |  you1 & I) == P(car3  |  you1 & I),
    ##
    P(host2  |  you1 & car1 & I) == P(host3  |  you1 & car1 & I)
)
#      min      max 
# 0.666667 0.666667
```

The well-known result is 2/3. You can also check the obvious result for `car3`, which is 0.

## Omitting some probability constraints

It is interesting to see what happens to the bounds of the target probability when some constraints are omitted. Suppose for instance that we omit the constraint\
`P(host2  |  you1 & car1 & I) == P(host3  |  you1 & car1 & I)`\
which states that we have no reason to believe the host would choose door No. 2 more than No. 3 or vice versa, when the car is behind door No. 1. The bounds of the target probability become


``` r
inferP(
    target = P(car2  |  you1 & host3 & !car3 & I),
    ##
    P(car1 & car2  |  I) == 0,
    P(car1 & car3  |  I) == 0,
    P(car2 & car3  |  I) == 0,
    P(car1 + car2 + car3  |  I) == 1,
    ##
    P(host1 & host2 | I) == 0,
    P(host1 & host3 | I) == 0,
    P(host2 & host3 | I) == 0,
    P(host1 + host2 + host3  |  I) == 1,
    P(host1  |  you1 & I) == 0,
    P(host2  |  car2 & I) == 0,
    P(host3  |  car3 & I) == 0,
    ##
    P(car1  |  I) == P(car2  |  I),
    P(car2  |  I) == P(car3  |  I),
    ##
    P(car1  |  you1 & I) == P(car2  |  you1 & I),
    P(car2  |  you1 & I) == P(car3  |  you1 & I)
    ##
    ## P(host2  |  you1 & car1 & I) == P(host3  |  you1 & car1 & I) # omitted
)
# min max 
# 0.5 1.0
```

In this case the constraints do not determine a single value for the probability that the car is behind door No. 2, but they still constrain it to be more than 1/2. The conclusion is that it is still best to switch door (if you want the car), but the credibility of winning is not 2/3.


## Variations

The internet offers many variations of the Monty Hall problem. They can also be solved with `inferP()`.

Let's take a "Monty Fall" variation as an example. There is the possibility that the host falls while going to open a door. We represent this with the proposition `fall`.

If the hosts falls, there is a non-zero probability, let's say 10%, that the host will open the door with the car after you choose door No. 1. We can express this by the constraints    
`P(host2  |  car2 & fall & you1 & I) == 0.1`    
`P(host3  |  car3 & fall & you1 & I) == 0.1`

Finally we still have no reason to believe that the falling host favours one door over another, when the car is behind the door you chose:    
`P(host2  |  you1 & car1 & fall & I) == P(host3  |  you1 & car1 & fall & I)`

The target probability is for the car being behind door No. 2, given that you chose No. 1, the host fell and opened No. 3, but there was no car there:    
`P(car2  |  you1 & host3 & !car3 & fall & I)`.

The result is

``` r
inferP(
   target = P(car2  |  you1 & host3 & !car3 & fall & I),
    ##
    P(car1 & car2  |  I) == 0,
    P(car1 & car3  |  I) == 0,
    P(car2 & car3  |  I) == 0,
    P(car1 + car2 + car3  |  I) == 1,
    ##
    P(host1 & host2 | I) == 0,
    P(host1 & host3 | I) == 0,
    P(host2 & host3 | I) == 0,
    P(host1 + host2 + host3  |  I) == 1,
    P(host1  |  you1 & I) == 0,
    P(host2  |  car2 & fall & you1 & I) == 0.1,
    P(host3  |  car3 & fall & you1 & I) == 0.1,
    ##
    P(car1  |  I) == P(car2  |  I),
    P(car2  |  I) == P(car3  |  I),
    ##
    P(car1  |  you1 & I) == P(car2  |  you1 & I),
    P(car2  |  you1 & I) == P(car3  |  you1 & I),
    P(car1  |  you1 & fall & I) == P(car2  |  you1 & fall & I),
    P(car2  |  you1 & fall & I) == P(car3  |  you1 & fall & I),
    ##
    P(host2  |  you1 & car1 & fall & I) == P(host3  |  you1 & car1 & fall & I)
)
#      min      max 
# 0.642857 0.642857
```
roughly a 64% probability that the car is behind door No. 2. You can check that this probability becomes 50% if there's a 50% probability that the falling host opens the door with the car.

An interesting aspects of such variations is to precisely translate the different verbal information into formal propositions and their probability constraints.



# References

Jaynes (2003): *Probability Theory: The Logic of Science* (Cambridge University Press) <doi:10.1017/CBO9780511790423>.

Jeffreys (1983): *Theory of Probability* (3rd ed. Oxford University Press) <doi:10.1093/oso/9780198503682.001.0001>.

Johnson (1924): *Logic. Part III: The Logical Foundations of Science* (Cambridge University Press) <https://archive.org/details/logic03john>.

Johnson (1932): *Probability: The Relations of Proposal to Supposal* (Mind 41(161):1) <doi:10.1093/mind/XLI.161.1>.

Hailperin (1965): *Best Possible Inequalities for the Probability of a Logical Function of Events* (Am. Math. Monthly 72(4):343) <doi:10.1080/00029890.1965.11970533>.

Hailperin (1996): *Sentential Probability Logic: Origins, Development, Current Status, and Technical Applications* (Associated University Presses) <https://archive.org/details/hailperin1996-Sentential_probability_logic/>.

Hailperin (2006): *Probability logic and combining evidence* (Hist. Philos. Log. 27(3):249) <doi:10.1080/01445340600616289>.

Pelletier, Hazen 2023: *Natural Deduction Systems in Logic* (Stanford Encyclopedia of Philosophy) <https://plato.stanford.edu/archives/spr2023/entries/natural-deduction/>.

Takeuti (1987): *Proof Theory* (2nd ed. North-Holland).

