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Turing and Decision Problem
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0:00
Today we are going to be talking about a problem in logic and how in solving that problem, Alan Turing almost inadvertently invented the modern digital computer.
0:12
So we start back at the beginning of the 20th century, where mathematicians had posed this problem - in logic we're interested in finding "Do these premises entail this conclusion?" So premises are the bits you start off with in an argument they are your -- the bits you know at the beginning or your assumptions and the conclusion is the bit you want to establish the bit that you reason to with your argument and we want to know is there a test that will tell us
0:39
yes for sure these premises do or don't entail this conclusion. Is there an automatic way of finding out whether they do or whether they don't? So that's the problem, it's called the decision problem. The mathematicians wanted to find out "Is there an answer to the decision problem for first order logic?" that's the kind of logic you learn in philosophy or mathematics at university so lots of mathematicians were trying to work out is first order logic decidable
1:09
that is can we automatically test whether the premises entail the conclusion Alan Turing was one of the first to discover that first order logic isn't decidable. To prove this it's really difficult conceptually because you have to be able to show no possible program can give you the answer but how do you do that how do you show something about every possible program you can't run through every program one by one but Turing came up with a brilliant solution his idea goes something like this
1:47
suppose we have a program and let's just draw it as a black box it's going to take some inputs and it's going to give us some outputs our program is going to solve some problem a problem like "Do the premises entail the conclusion" we ask it a question and it will give us an answer yes or no now here's another question we can ask let's look at all of those possible programs and we're just thinking of them as black boxes at the moment we might
2:21
want to know is this program given a certain input going to give us an answer or is it going to trundle on forever and never give us an answer that is is it going to halt or is it not going to halt eventually so think about your computer running you want it to give you an answer of whether it's a good answer or a bad answer it's better than no answer. No answer would mean the computer trundles around forever and ever in a loop and you would just never know whether it's going to
2:51
finish today tomorrow or never so halting is good so there's another question we can ask given some program and some input will it ever halt? Now it turns out that our logical problem "Do these premises entail this conclusion?" is very similar to this halting problem in fact if we can solve the logical problem then we can solve the halting problem will this program halt on this input so the clever part of Turing's proof is
3:27
to show that it's impossible for any machine however clever it is to solve the halting problem that is to tell us whether a given machine with a given input will halt or not - and here's how he did it let's suppose we've got a machine or a program that solves the problem for us it solves the halting problem don't worry about how it works let's just think of it as a black box taking the description of a machine and an input and giving us an answer
3:58
yes it will halt or no it won't halt just suppose that's possible call that machine "h" for the halting problem - if you give me that machine I can transform it into a different machine like this I stick some extra bits on it so that if it gives me a yes answer I make it loop forever without ever stopping if it gives me a no answer on the other hand and it's going to halt straight away let's call that big machine the whole thing "h+" now here's another question we can ask what happens if I
4:35
feed the whole machine into itself so i'm going to put h+ in here and h+ in here so the question I'm now asking is I'm feeding h+ into itself so i'm asking the question "Does h+ halt given input h+?" and here's where it all goes wrong because if h plus does halt we get a yes answer but then it loops forever so it doesn't halt on the other hand if it doesn't halt we get a no answer but then it halts so if it does halt then it doesn't halt
5:17
but if it doesn't halt then it does halt. Either way we get a contradiction it's a paradox but what that shows is we started off assuming that we can solve the problem we've ended up with a paradox so our assumption was bad it turns out there's no possible machine no possible program that solves the halting problem the really clever bit about Turing's idea is it doesn't matter what kind of program our machine is. It doesn't matter whether it's an abstract algorithm
5:49
whether it's a real computer, a physical computer it doesn't matter what it is we've prove that no such program as possible
6:00
Turing as part of his argument had to say a little bit about what's going on in these black boxes //DFB: and the idea is that every card represents an instruction in the Turing machine
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