Mackie on Leibniz's Cosmological Argument

Mackie does a decent job of summarizing what seems to me the classic objections to Leibniz's argument. It is formulated like this:

• "He assumes the principle of sufficient reason, that nothing occurs without a sufficient reason as to why it is so and not otherwise. There must be then a sufficient reason for the world as a whole, a reason why something exists rather then nothing. Each thing in the world is contingent, being causally determined by other things: it would not occur if other things were otherwise. The world as a whole, being a collection of such things, is therefore itself contingent. The series of things with events, and their causes, with causes of those causes and so on, may stretch back infinitely in time; but if so, then however far back we go, what we have is still contingent and therefore requires a sufficient reason outside the series. That is there must be a sufficient reason for the world other then the world. This will have to be a necessary being, which contains its own sufficient reason for existence." (pg 82)

Mackie launches two specific criticisms: "How do we know that everything must have a sufficient reason" and "How can their be a necessary being, one which contains it's  own sufficient reason?" Mackie discards the notion that the second criticism depends on the ontological arguments because this argument leads one to think that the contingency of the world leads one to think that a necessary being exists, not that this being exists in a logically necessary fashion. But even though this argument doesn't descend into the Ontological Argument that doesn't mean that the interrogation of the legitimacy of this necessary being nonsense shouldn't be interrogated. In the first place neither me or Mackie really knows what it means for a being to exist necessarily, Mackie offers two answers:

• For something to exist contingent is for it to not exist if things had been otherwise, for something to be necessary is for something to exist even if things had been otherwise
• But what then our are grounds for thinking that the world as a whole could have been otherwise?
• It doesn't seem to me like this is a particularly strong objection, because it is quite conceivable that the big bang could have never occurred. But at the same time it's not clear to me what it means for things to be otherwise in relation to the big bang or some other cosmic event.
• Something is contingent if it might not have existed, for something to exist necessarily is for something to exist and not be the case that it might not have existed.
• But once it has been conceded that it is not logically impossible that this being exist, how then can it be said that it could not be the case that it might not have existed?
• This seems like a much better objection, I will have to go read some material written by supporters of this type of argument and see what they have to say about necessity.

Now in regard to the very first question it's seems very hard to support this principle of sufficient reason. Although it seems plausible that contingent beings inside operating natural laws have reasons for their existence is it really true to say that the natural laws themselves have a reason for existence?