In deduction, we use logic and generalisations to come to conclusions, or deductions about a thing or person. Many people do this subconsciously, little things like what hand someone writes with or if someone gets engaged, but never takes it to its full extent.
Of course, we all know Sherlock doesn’t use deduction. He uses a mixture of all three types, Deduction, Induction and Abduction, things we are writing posts on right now.
Deduction, however, is a science, and as with many others, has laws, things that cannot be broken and still have your conclusion come out as true. These laws are easy to learn and can help stamp out any incorrect deductions you may make, just by running them through the three of these:
/THE LAW OF DETACHMENT
From Modus ponens, the Law of Detachment basically means if a conditional statement and a hypothesis are made, a conclusion must fit both to be true.
The Latin, Modus ponens, means “the way that affirms by affirming” which basically explains the entire concept. Put simply, “if X implies Y, and X is true, so must Y”
- X=Y is the conditional statement;
- X as the hypothesis;
- Y as the conclusion.
An example of this is:
- If someone is right-handed, they will write with their right hand
- Someone is writing with their right hand
- The person is right-handed.
Pretty simple concept, and very useful.
/THE LAW OF SYLLOGISM
Syllogism comes from Greek, and it means conclusion or inference. You have probably seen an example of it before, in the form of:
All men are mortal
Socrates is a man
Therefore Socrates is mortal
So what does it mean? Well, simply, a hypothesis of a statement combined with the conclusion of another. In the example, hypothesis 1 (H1) is “all men are mortal” hypothesis 2 (H2) is “Socrates is a man” and if you combine H1 with the conclusion of H2 then we reach the conclusion of “Socrates is mortal” which is correct, in this case. Simply:
However, this law isn’t always true. There can be times when your conclusion isn’t valid, for example:
Some televisions are black and white
All penguins are black and white
Therefore, some televisions are penguins
Clearly, this is wrong. A way around this is to not form hypotheses that have no conclusive proof, only use ones that have been accepted by the majority of people, and don’t try to combine incompatible hypotheses, and also, the law itself does not have flaws, the logic is there, and it’s your responsibility to make sure YOUR logic is sound when you use this law.
/THE LAW OF CONTRAPOSITIVE
This has the longest title, but is the easiest concept to understand. Contrapositive also comes from Latin, this time Modus tollens, meaning “the way that denies by denying” the concept basically states that if a conclusion is false, so must the hypothesis.
Again, an example:
If I am lonely, there is nobody around
x, y
There are people around;
I am not lonely.
x = y
Opposite of x = Opposite of y
Another pretty simply rule, but you can see that it’s true.
And that’s it, the three laws of deduction: detachment, syllogism and contrapositive. You do not need to learn them all, but if you are planning on making deductions that heavily involve hypothesises, then I suggest asking an effort.
Technically, these are laws of logic, but many places I researched them put them down as laws of deduction, so whilst the name of them is to be debated, I’m confident that they work and are sound.
Any questions or corrections feel free to send an ask.
XW
