Simple enough question, and very much related to the topic.
According to traditional Jewish sources, on what basis should people believe that there's a God?
How does a Chasid know that G-d exists? He has Emunah.
How does a Litvak (a.k.a. Yeshivish Jew) know that G-d exists?
The Rambam says so, and the Raavad doesn't argue. :o)
It may have been formulated as a joke, but it is a serious answer. We know that G-d exists because our sages have transmitted this fact. We can also experience G-d's Providence in our lives on a continuous basis, but that's a beneficial side-effect of knowing, not how we know. – user1095 Feb 1 '12 at 11:46
Assuming we all exist, think, know and interact with our actual surroundings etc.
The first class of Divine Proof is the Ontological proof. It goes basically like this:
God as a concept is perfect.
Perfect things must have the quality of existing, else they wouldn't be perfect.
Hence, God exists.
This doesn't really convince anyone of anything.
Many philosophers have spent a lot of time tweaking this proof, but in the end I don't think anyone else really cares about it.
Everything that moves was moved by something else.
There cannot be an infinite regress of events.
Hence, there exists a Prime Mover and we call Him God.
First off, you could just say that there can be an infinite regress of events.
Secondly, in the current proof what can God tell you to do? Move? Not so powerful...
Two avoid the second problem, some have changed "move" to other powers and given those to God.
Alternatively, some focus on the fundamental nature of the proof which is that there exists a being external to the general rules of science. They will argue something like this:
Morality* is eternally true independent of circumstances.
Hence, there exists some entity outside of the natural world and we call it God.
Rationality or some other fundamentally True concept as you please.)
This doesn't really get exactly the God we want, but it is better if you assume Morality/Rationality/etc. is eternally True independent of circumstances.
[Some aspect of the world] is very complex.
Hence, it must be designed as such by a Higher Being whom we call God.
It might not be. Everyone agrees this is not a strict proof. [BTW this is why people are so opposed to Darwin, not the textual issues from Genesis 1]
Err.. Find something REALLY REALLY complex.
There is no strict philosophical proof for God; however, there is no strict proof against the existence of God either. You need to evaluate the evidence and BELIEVE.
Sources: Philosophy classes and personal research on the subject.
To my knowledge, the only "argument" for the existence of God given in the Torah itself is that He directly revealed Himself to us at Sinai:
Deut. 4:35 "Unto thee it was shown, that thou mightest know that the LORD, He is God; there is none else beside Him."
Deut. 5:4, "The LORD spoke with you face to face in the mount out of the midst of the fire."
In other words, direct experience does not require philosophical proof.
Rabbi Jacob Emden expanded this argument into our own time, saying that, "When I consider these wonders [of the survival of the Jews in exile], they appear greater to me than all the miracles and wonders that God did for our ancestors in Egypt, and in the wilderness, and in the land of Israel."
All philosophical arguments for the existence of God made in traditional sources are only intended to reinforce this basic experiential knowledge that is the heritage of the Jewish people. While these arguments can serve to shore up our beliefs against challenges, many sources see these arguments as helping us acheive a more personal, immediate connection to God.
The most common such arguments found in Jewish works are:
The Argument from Design - Many aspects of the natural world appear to have been been designed with intelligence and intent.
The Cosmological or "First Cause" Argument - What set the world moving? Where did it come from?
The famous "Kuzari" argument (which is also found in the writings of R' Saadia Gaon and Maimonides), that the Sinai revelation was a historical event witnessed by the entire nation. (This argument is basically just an extension of the Biblical "argument" that is intended to enable us to rely with confidence on our historical tradition.)
In my personal opinion, the various philosophical arguments for the existence of God are mainly useful for countering Hume's arguments against miracles. Briefly stated, he argues that no testimony of a miracle should be believed unless the falsehood of the testimony would be more improbable than the miracle itself. It follows, therefore, that one's ability to accept the testimony of the Jewish people's historical experience of miracles has an inverse relationship with the degree to which you think miracles are improbable.
All of the classical arguments for the existence of God are, fundamentally, arguments that we can perceive an element of the supernatural in the natural world itself. Thus, each such argument makes the possibility of miracles more plausible. At some point, it becomes more likely that Sinai Revelation really occurred than that it was made up (which, per the Kuzari argument, is very unlikely). Once you reach that point, then you have the Sinai Revelation to rely on for everything else.
Rabbi Hirsch says a "proof is in the pudding" type of argument, namely if you keep the mitzvos it will become clear to you through your experiences that you are involved in something higher. It need not be explained - you will feel it, if you are keeping the mitzvos properly. This experience is an experience of G-dliness, and is the best "proof" of His existence.
This is quoted in Dayan Grunfeld's intro to Horeb.
I'm a terrible one for sources, however as far as I am aware there are four ways in which Judaism knows that Gd exists.
The first method is brought in the midrashim regarding Abraham. There are two stories that I know of regarding how Avraham knew that G-d existed.
Story 1: (Thank you @Monica Cellio)
Abraham was walking along the way and saw a building that either (had a light in it's window, or was on fire), Avraham looked at the building and said, "Can that house have no master?, surely there is a master of the world."
I have seen the midrash explained in two ways based on how you translate the line. Either, a building with a light on must have someone who lives in, and so with our world, life exists, so someone must have created it. Or, a house would only be on fire if someone owns it and an enemy wished to destroy it. Since there is evil in this world, and there is constant entropy, the fact that things get "built" means there is a creator. In modern language you might describe this as entropy vs evolution. However as with all midrashim, it could be undertood differently by each person.
Story 2: Abraham was sitting in the sun, when suddenly a cloud blocked the Sun. The Cloud must be stronger than the sun, then a wind came, and the wind moved the cloud, so wind is stronger than clouds, then the wind came to a mountain and was blocked, so a mountain is stronger than wind, then a river was seen carving through the mountain, so water must be stronger than mountains, then the sun dried up the water, so the sun must be stronger than water... This cycle lead Abraham to believe that there must be a force outside of the cycles of nature which is strongest of them all, and runs it all, and that force is Gd. In modern language you might say, "That which breathes life into the equations." However as with all midrashim, it could be undertood differently by each person.
** Edit: It seems that the original midrash talks about man being better than spirit because of a wordplay, so I'm not sure where I heard this version from, but the basic concept is the same.
The second method in which we know G-d exists, is in the fulfillment of prophecies and being able to see the hand of G-d in history. This is first mentioned in the Torah when G-d describes himself as the Lord Your G-d who took you out of Egypt. This was a common way for the Rabbis of the Talmudic and Gaonic period to relate to G-d, most notably the book the Kuzari written by Yehuda HaLevi. Various prophecies have come true over the centuries, the latest of them being the return of the Jewish people to the land of Israel.
The third method in which we know G-d exists is Rambam's principle of knowledge and the methods of books such as Derech Hashem which aim to prove G-d logically as either the first cause of all things, or show G-d as a necessity in some other logical manner. This method became popular in the middle ages, and has only grown since then. This method has become the most popular over the centuries, to the point that now most Jewish understandings and proofs of Gd are no different from any other theistic philosophy.
The fourth method of knowing that G-d exists is through personal experience and prayer, where a person feels a close relationship with his creator. This is most common in circles influenced by the Hassiduth movement from the 18th century.
Since Kant's Critique of Pure Reason, there has been little faith in any philosophical proof of theological and metaphysical claims. But then, despite the misnamed "Kuzari Principle", this is R' Yehudah haLevi's point in much of the first section of the Kuzari as well.
The Kuzari opens with the king having a series of dreams in which an angel tells him, "Your way of thinking pleases G-d, but not your way of acting."
The king believes because of his experience of the Divine, not because of a proof.
Similarly, the chaver tells the king in 1:13: That which you describe is religion based on speculation and system, the research of thought, but open to many doubts. Now ask the philosophers, and you will find that they do not agree on one action or one principle, since some doctrines can be established by arguments, which are only partially satisfactory, and still much less capable of being proved.
And later in their dialog, 1:63: “There is an excuse for the Philosophers. Being Grecians, science and religion did not come to them as inheritances.”
R’ Gil Student posted the following quote from Louis Jacobs, We Have Reason to Believe, pp. 25-26, 28-30 on Hirhurim's article on the Proofs of God :
Since Kant, these proofs [of God’s existence] have been heavily assailed…. Many theologians, nowadays, accept the validity of these refutations and admit that there can be no proof of God in the sense that there can be no proof of a mathematical formula… But they go on to remark that we can be convinced of a thing beyond of a shadow of a doubt by means other than that of mathematical proof. There is no such proof, for instance, of the existence of other human beings beside ourselves, yet we are convinced that they do exist… In other words a distinction must be drawn between proof and conviction — proof is one of the ways to conviction but there are other ways, too…
Many have arrived at this conviction as the result of a personal experience which convinces them that God exists. These men would rule out of court the very discussion of whether God exists, for, they would say, if a man is truly in love he does not ask himself if he is in love. The experience of God’s Presence is sufficient…
Other thinkers, again, hold that though each of the traditional proofs in itself is unconvincing, taken together they are convincing… Granted that the proofs carry no weight as evidence, they are indications and as such have the power of supplementing each other…
What it all amounts to is this, that while the existence of God cannot be proved if we start from the beginning, none of us do, in fact, start from the beginning. We are presented with two alternative beliefs about the ultimate reality and we have to choose between them. According to one view God exists–it is He Who created us, Who fashioned our minds and implanted the moral sense within us so that we are capable of recognising beauty, truth and goodness and fighting ugliness, falsehood and evil. In this view the difficulty is how to account for the existence of evil. According to the other view there is no God… In this view the difficulties are how mind came from matter, how life emerged where there was no life before, how the universe itself came into being, how the good is possible of realisation and how man came to strive for it–how man as a tiny part of the universe came to pass judgment on it?
Similarly, a more contemporary philosopher R/Prof Shalom Carmy (of YU) wrote on Avodah:
People who throw around big words on these subjects always seem to take for granted things that I don’t.
The people who keep insisting that it’s necessary to prove things about G-d, including His existence, seem to take it for granted that devising these proofs is identical with knowing G-d.
Now if I know a human being personally the last thing I’d do, except as a purely intellectual exercise, is prove his or her existence.
Ever notice how looking at a working hand is so much more compelling of an argument from design than the actual Argument from Design?
I believe because the experience of Shabbos as per the halakhos humans interpreted and legislated from the system He gave us compellingly tells me He's there. I believe because I study His Torah, and the sheer aesthetics of it tells me He's there. I hear Him in my struggles with kashrus and taharas hamishpachah and in the beauty of a flower.
This is why Shabbos is described in the Torah (and Qiddush) as being "between Me and the Children of Israel, a sign forever, that in six days Hashem made heaven and earth, and on the seventh day He 'rested'."
Intelligent Design is the classic and most powerful argument for the existence of God as scripture says "from my flesh I shall see God" (Job 19:26)
(see the the treatise shaar bechina of chovos halevavos which delves into this at depth or the book "The Universe Testifies" by Rabbi Avigdor Miller. These works not only address the intelligent design of life forms but also at the structure of the universe as a whole - everything has a function. see there.)
The rise of the random evolution theory last century caused the argument to lose some impact but the tide has been turning.
With the advance of microbiology the argument has become much more powerful as the famous philosopher Anthony Flew, who for many decades flew the flag for atheism and then reversed said:
"I think that the most impressive arguments for God’s existence are those that are supported by recent scientific discoveries. I’ve never been much impressed by the kalam cosmological argument, and I don’t think it has gotten any stronger recently. However, I think the argument to Intelligent Design is enormously stronger than it was when I first met it."... FLEW: Absolutely. It seems to me that Richard Dawkins constantly overlooks the fact that... the findings of more than fifty years of DNA research have provided materials for a new and enormously powerful argument to design.
See also the book Nature's Destiny by Michael Denton which shows that the universe itself and the laws of physics, chemistry, biology, geology all appear to be uniquely fit for homo-sapien like life as it exists on earth.
Here a few of the proofs I collected a few a years ago (all from major scientists and professors by the way)
Of course, I have many stored in my computer somewhere.
There is the "cosmological" argument of King David in Tehilim 19:1-4, where creation itself "bears witness" to the existence of G_d:
The heavens are telling of the glory of G_d
I'll admit it now, I am biased in my belief that the traditional scriptures (i.e., the 24 books of the Hebrew Bible) trump any and every other source of traditional authority.
To be sure, virtually every other traditional source of authority has good and helpful things to offer, and those who are familiar with such sources should always feel free to mine them for nuggets of truth that can benefit both them and the larger community of faith (and even the community of non-faith!).
There is a hierarchy, as it were, of authority, and God's Word trumps all other authorities.
Rambam Hilchos Yesodei Hatorah Chapter 1
Halacha 5 This entity is the God of the world and the Lord of the entire earth. He controls the sphere with infinite and unbounded power. This power [continues] without interruption, because the sphere is constantly revolving, and it is impossible for it to revolve without someone causing it to revolve. [That one is] He, blessed be He, who causes it to revolve without a hand or any [other] corporeal dimension.
It is obvious. How else could such a world come to existence?
Not speaking the fact that (at very least) 600.000 men witnessed the miracles on exiting Egypt, crossing the sea, hearing Gcd and getting Thora at the Mount Sinai, eating the mann forty years in the desert...
Not entering in detail that, precisely this could be the mitsva: to be realist, and admit the truth.
ניכרין דברי אמת.
Written Torah is essentially the proof of God existence. Science (Big Bang) also confirms that all came out of 1 source. Thus there is God and there is only 1 real God. Other gods are not real.