Briefly, 'Ockham's Razor' says entities should not be multiplied unnecessarily - in other words, no explanation should include unnecessary steps or the existence of unnecessary components; the most parsimonious explanation in competing hypotheses is most likely to be the correct one.
First the background:
William of Ockham (also Occam, Hockham, or several other spellings; c. 1288 – c. 1348) was an English Franciscan friar and scholastic philosopher, who is believed to have been born in Ockham, a small village in Surrey. He is considered to be one of the major figures of medieval thought and was at the centre of the major intellectual and political controversies of the fourteenth century. Although he is commonly known for Occam's razor, the methodological principle that bears his name, William of Ockham also produced significant works on logic, physics, and theology. In the Church of England, his day of commemoration is 10 April.Which is a long-winded way to state the basic principle, "keep it simple, stupid".
Source: Wikipedia - William of Ockham
William of Ockham... is remembered as an influential Roman Catholic philosopher and nominalist, though his popular fame as a great logician rests chiefly on the maxim attributed to him and known as Ockham's razor. The term razor refers to distinguishing between two hypotheses either by "shaving away" unnecessary assumptions or cutting apart two similar conclusions.
This maxim seems to represent the general tendency of Occam's philosophy, but it has not been found in any of his writings. His nearest pronouncement seems to be Numquam ponenda est pluralitas sine necessitate [Plurality must never be posited without necessity], which occurs in his theological work on the 'Sentences of Peter Lombard'.
The words attributed to Occam, entia non sunt multiplicanda praeter necessitatem [entities must not be multiplied beyond necessity], are absent in his extant works; this particular phrasing owes more to John Punch. Indeed, Ockham's contribution seems to be to restrict the operation of this principle in matters pertaining to miracles and God's power: so, in the Eucharist, a plurality of miracles is possible, simply because it pleases God.
This principle is sometimes phrased as pluralitas non est ponenda sine necessitate [plurality should not be posited without necessity]. In his Summa Totius Logicae, i. 12, Ockham cites the principle of economy, Frustra fit per plura quod potest fieri per pauciora [It is futile to do with more things that which can be done with fewer].
Source: Wikipedia - Ockham's Razor
So how does that get rid of gods and other hypothetical supernatural entities from the explanation for natural phenomena?
Take, for example, a common theological argument that the Bible or Qur'an were dictated or inspired by a god and that their survival over a long period of history is due to their divine status - which is itself evidence of their divine status.
- The circularity of the argument;
- The fact that there are many other surviving documents and inscriptions not claimed to be divinely inspired, some older than the Bible and Qur'an;
- The fact that it can be used for literally any old books;
- The fact that there appears to be no particular date before which divine intervention is needed to ensure conservation but after which supernatural intervention need not be hypothecated.
- People considered them sacred and so looked after them and made copies of them.
- A god told people they were sacred and that they should look after them and make copies of them.
We could still imagine that there is a set of laws that determines events completely for some supernatural being, who could observe the present state of the universe without disturbing it. However, such models of the universe are not of much interest to us mortals. It seems better to employ the principle known as Occam's razor and cut out all the features of the theory which cannot be observed.The difference between 1 and 2 of course is that 2 includes a god, yet this adds nothing to an already complete explanation and indeed it had to include 1.
Stephan Hawking, A Brief History of Time
Including a god simply adds an additional entity. It also includes an entity for which no explanation is possible and which is untestable and unfalsifiable. Indeed it is now necessary to explain something vastly more complex than the phenomenon originally being explained - the survival of (copies of) old documents.
To reach a complete explanation which includes a god we also need to produce independent evidence for the existence of this god and an explanation of its origins and modus operandum. How exactly did it communicate this instruction and where is the evidence that it was ever conveyed? To whom and when?
So, in addition to the god the explanation now includes a whole lot of new entities, all needing to be there to justify including a god in the first place, when the natural explanation in 1 was perfectly adequate.
This is precisely what religious apologists do when they insert gods into gaps in scientific theories, real or imaginary. Including an infinitely complex god in any explanation simply because you want it to be there invariably adds an infinite complexity to the explanation when the natural explanation, whenever it has been found and a god has been evicted from yet another gap, has always turned out to be relatively simple and rational in comparison to a hypothetical god.
Ockham's Razor, properly applied, will invariably pare gods away from any explanation because the inclusion of gods multiplies entities infinitely and unnecessarily. There is never an excuse for insisting an entity be included in any explanation just because you like it and want it to be included.
In pluralitas non est ponenda sine necessitate, necessity has nothing to do with your superstition, your need for an imaginary friend, your need to excuse otherwise unacceptable attitudes and behaviour and/or your need to earn a living selling superstitions to gullible and vulnerable people. And it has nothing to do with your inability to accept that your mummy and daddy could have been wrong.