You will not arrive at philosophical proof of G-d’s existence because, philosophically, G-d does not exist. There is no definition of the word ‘exist’ under which G-d can be known to exist. Everything by which we define this word, everything we know for certain to exist, is in some way caused. G-d Himself, however, is not caused, and is free to be completely different from everything we know. Thus, given G-d, defined as the uncaused cause of all else, we cannot say G-d exists with the same existence as anything else we know. If the word applies to Him, it applies only as a personal name, not as a category.
This led philosophers such as the Rambam/Maimonides to call G-d the ‘metzius bilti metzius nimtza,’ roughly ‘the Existence without an existing existence.’ Or in other words: the Rambam says we cannot affirmatively say He exists (for the reason explained in the previous paragraph). Still, we are forced to say He lacks non-existence, because if He does not at least lack non-existence, how can He lend the existence with which we’re familiar to everything else? One cannot give what one does not possess; if G-d does not exist, how can the universe? So the Creator thus dwells in a third category, sharing the same definition of ‘existence’ with the universe enough to grant its existence, but utterly different enough to not be captured by that word. To creations such as us, the way He is both these things is utterly mysterious.
Since, per philosophy, He must exist at least as much as the universe does, maybe we can reach Him with proof after all. If philosophy was the only path open to us, we would say that a theoretical proof can reach G-d Himself, since G-d Himself is that which lends existence to the entire universe. The existence of the world is where philosophy begins, and the necessary existence of G-d, the source of the world’s being, is where such a proof would end.
The reason I say G-d is beyond the reach of philosophical proof is that there are other paths to reach the Creator, namely revelation. If G-d Himself reveals His own nature to humankind, all bets are off. In particular, if G-d tells us He can somehow cause things to exist the way our universe does, without having to Himself participate in that form of existence at all, it throws off our whole previous calculation. After all, the rule we used to demonstrate that G-d must exist at least as much as the universe, the law that you cannot give what you do not have, is an assumption itself based on how things work within our world. Philosophically, we cannot escape this rule; it seems to be built in the logical fabric of our reality; 0 + 0 cannot equal 1. But since, as even philosophy acknowledges, G-d exists in a categorically different way than everything else, that is, He has no cause, maybe this rule does not apply to Him either. All we need is a good reason to place Him beyond the rule.
Revelation serves this purpose. The Torah tells us G-d creates the universe ex nihilo, something from nothing, that is, not as a direct extension of His own existence, but with some kind of causal gap that by definition is impossible for us to understand. The universe’s being does not have to be some kind of subset or direct result of His being at all, because He can cause the universe while remaining at an infinite causal remove from it. The Torah tells us so.
With this piece of information, we must revise our conclusion: G-d’s existence is unknowable to the creation, and nothing compels Him to have anything in common with the universe whatsoever. Therefore, there cannot ultimately be a philosophical proof of G-d’s existence. At least, no unaided philosophical proof will land on the same G-d we know through revelation. Any given philosophical proof will take some created existence as a prerequisite, work its way back under the laws of logic that bind our reality, and conclude at the very least with a Creator who explains the created things from which we are arguing. By revelation, however, we know that G-d is not, in fact, compelled to explain any creation. He can cause it without being a causal explanation for it. This is what ‘creation’ means as the word is used in the first verse of Genesis, and it is not something even the greatest philosopher can comprehend, for all philosophy is at root a study of explanation.
So, the philosophical proofs are not proofs for G-d. What, then, are they proofs for? After all, for reason to so insistently converge on something that so many have called G-d, a necessary first cause for all that exists, cannot just be an accident! And it isn’t. The proofs reach the first cause of all that exists, the necessary first existence that causes all other existence. If this is not G-d Himself, the G-d known with the help of revelation to exceed all logic and all proof, it can be G-d as He descends to exist before creating, as it were. In other words, what the philosophical proofs point to is not G-d per se, but rather G-d-in-the-act-of-creation.
G-d-in-the-act-of-creation is more readily understood under the Kabbalistic doctrine of divine emanation than under the philosophical rubric. This fits perfectly. The emanated G-d-as-first-cause is anterior to all of philosophy’s tools (which all deal with existence under existence’s rules). Philosophy, per the Torah, cannot understand how its own cause comes into being; that realm is shut to the eyes of the mind, existing beyond all the rules we know to rule the created world.
This divine act of descending to create satisfies all of the philosophical characteristics of G-d when viewed by philosophy from the bottom up. That is, there is nothing about it that breaks the classical proofs. For example, everything that exists depends on it, and it depends on nothing that exists. It is absolutely simple and uncaused. The only sense in which it is complex and caused is the sense in which it relates to G-d per se who precedes it, and this relationship is itself ungoverned by the laws of logic or the usual definition of the words. Everything traditionally said about G-d is correctly ascribed to G-d-as-He-descends-to-create.
One can see why Kabbalah, to the unstudied, may seem to introduce multiplicity, G-d forbid, to the Creator. But in fact, what is here described is not a multiplicity at all, but a unity. It is merely not a unity that may be precisely philosophically described. This is why Kabbalah is not a violation of the codified theology in the Rambam’s Mishne Torah, which describes G-d Himself in all the familiar terms, the Being Who Brings All Other Beings Into Being, the Knower, Knowledge, and Known, etc. All of these terms indeed describe G-d Himself, for the ‘two G-ds’ described in this essay are not two G-ds, G-d forbid, but absolutely One G-d. ‘Hashem and Elokim are all One.’
The main reason this makes some Jews nervous is that it sounds to them, on the surface, like a Christian doctrine, G-d forbid. Further study, however, reveals not only that the Jewish notion of the Divine emanation is substantially different from the Christianity l’havdil, but also that Judaism does not reject Christianity for any theological doctrine per se but rather for its abrogation of the Law. Since it is the Law itself that opens up for us the nerve-wracking ‘non-rational’ notion of G-d, the Jews who still today irrationally oppose the Kabbalah may sleep easy. Those who reject the eternality of Moses’ prophecy have no justification, Judaically, to go tampering with G-d’s unity.
So, nu, what is the best philosophical proof for G-d-in-the-act-of-creation? Good question. First of all, the classical proofs are better than many assume and deserving of study, though given our lengthy introduction, they will not lead to the satisfaction of catching G-d by the toe (so to speak). They are especially useful as contemplations of the way the Creator is implicit in His Creation, or more accurately, the way the apparent independence of creation really, upon some thought, gives way to inner structures of dependence and, ultimately, nothingness. Really, to the Jew, proofs for G-d are proofs for the creation, demonstrations of the relationship with the creator inherent to the creation’s logic.
To this end, if you’re really serious, you should check out some modern scholarship on the proofs of the medieval or scholastic philosophers. My personal favorite (the one I find most intellectually intuitive and easiest to explain) is the Neoplatonic proof based on unity, but as the astute reader will find, almost all these proofs are variations on one another and work much the same. It is worth investigating why many of these are widely considered today to be philosophically irrelevant, and why, according to the latest and strongest arguments, they aren’t.
If by ‘best’, you mean the one most central to Judaism, it is worth noting that Abraham, the first Jew, discovered G-d’s existence after being raised by idolators through something very much like a teleological proof. As the sages teach us:
G-d said to Abram, ‘Go forth from your land…’ (Genesis 12:1)
Rabbi Yitzchak opened and said: ‘Listen, daughter, look, and incline your ear, and forget your people and your father’s house.’ (Psalms 45:11)
Rabbi Yitzchak said: this may be compared to a man who was traveling from place to place when he saw a a castle aglow. He said, ‘Is it possible that this castle lacks a person to look after it?’ The owner of the building looked at him and said to him, ‘I am the master of the castle.’ What happened with Abraham our father was similar. He said, “Is it possible that this universe lacks a person to look after it?’ The Holy Blessed One looked at him and said to him, ‘I am the Master of the Universe.’
That is, Abraham recognized in the purpose inherent to the creation that the purpose must point to Someone beyond the creation. The Tzemach Tzedek writes that in this brief Midrash from the sages are implicit the lengthy teleological proofs of the Rambam and the Ralbag. For Jews to understand their own father, Abraham, they may need to rediscover the lost doctrine known as ‘telos’ (or ‘tachlis’), the inherent purposes of things, which has been banished from the modern world. Do not believe too quickly the claim that science has ‘disproved’ this ancient wisdom…
None of these proofs, however, speak to my heart. My life has played out differently—I arrived at the G-d of the Torah first, and only then became interested in proofs. To my heart, there is only one ‘proof’. Someone has summarized it nicely:
The major premise of the argument is that ‘every natural or innate desire in us bespeaks a corresponsing real object that can satisfy the desire.’ The minor premise is that ‘there exists in us a desire which nothing in time, nothing on earth, no creature, can satisfy.’ The conclusion is that ‘there exists something outside of time, earth, and creatures which can satisfy this desire.’
There is a reason the biblical story of Abraham does not include his early philosophical discovery, but rather begins with G-d’s revelation and the command, ‘Go forth.’ Judaism is not a philosophical religion, but rather a religion that may find some use for philosophy. The last time Judaism was truly philosophical was before the Torah was given, when a young boy in Sumeria decided the smash his father’s idols and invent something he thought was new, the worship of an ultimate G-d, a necessary G-d. The Torah, speaking to his descendants, does not need to prove anything, nor could such philosophizing even point to G-d.
Good thing we were there at Sinai, you and I…