Once my friend got locked out of her flat, and I called a locksmith, who demonstrated that her front door locks (both the navtal and the yale door lock) could be picked within a matter of minutes! He made a profound observation...Madam, these locks are only for YOUR self-satisfaction!
So actually I think shutting the door and locking it is not to deter ..er...determined thieves, but to deter the casual ones, and the rest..for my own mental satisfaction.
So..the real security exists (mainly) in our perception of it....and resides in the general network of amorphous trust that exists between the people around me. I am fairly sure that though someone can, if s/he wants, break open my front door within minutes, it won't happen within the apartment building. My neighbours opposite are away, and I do check casually that their door is properly locked...
Also, I think that I must assess the value of what I have to be protected, how important it is to me, and what the replacement-ability (er, you know what I mean) and the replacement-value would be, and take measures accordingly. Jewellery cannot be stored at home; it must be kept in a bank locker, for example. Neither should large amounts of cash be kept at home. Credit cards and cheque books should generally be kept locked up even within the home...and so it goes.
And sometimes, I do use two reasonably known entities as a check on each other. Right now, the building electrician (whom I have known for several years) is working in my home. And I need to step out to buy vegetables. So when my maid (whom,also, I know very well, including exactly where she lives) comes to clean the flat, I will tell her, "just keep an eye out, Prakash is working", and go out for about 20 minutes and get my work done. Each will keep an eye on the other (or at least, I hope that they will not form an unholy and rapacious alliance to loot me!) and I too will be able to step out for a bit without worry. I use my knowledge of the risks, or lack of them, and construct my security according to that. I know that no security can withstand the determined security-breaker (so many novels and films are based just on this opinion!), but I trust that the security will hold up to a reasonable level. What is reasonable, though? That...varies from person to person.
I think internet security is also similar to this...one takes a reasonable level of precaution against being spammed or hacked...and trusts that the rest will go well, too.
What say, kalyan? If everyone did this, security firms might not come into existence at all...obviously, this solution will not work for everyone!
The best security systems, though, are ones that rely what is unique to the user: fingerprints, for example, or iris-of-the-eye. I can never understand why passports do not have the fingerprints of the holder, it is such an easy and unique parameter to have and the passport holder can never say s/he hasn't brought hes fingers along! And they could be so easily compared, too. In fact I am amazed that such a simple way of eliminating the use of stolen passports (forged passports with matching fingerprints are of course possible) is not carried out...