It is mentioned in the CPR. But if you're hoping for something in the affirmative, then you should look for another writer.

(Critique B621)

Who would you recommend? I am interested in both and all sides of arguments.

Yes and no at the same time. I love Kant, but we all still argue over interpretation, and interpretation is a big part of Kant scholarship. So yes, it's interesting. But if you haven't read much else in terms of philosophy, then no. It's dense and difficult and could turn you off. If you have that "spark" then that's not going to stop you, but there are definitely other interesting philosophers who talk on the same topics who aren't as dense.

Who — Corvus

Hume's Dialogues Concerning Natural Religion is key text for a lot of modern chitchat on the topic.

For attempts to argue for God's existence, you could do worse (at least in my opinion) that to read Descartes' Meditations (from memory the arguments for God's existence are in meditations 3 and 6 but you should read all 6 anyway - they are by no means as difficult and dense as Kant). Then move on to Berkeley's Three Dialogues Between Hylas and Philonous.for a different take. Spinoza presents a variation of the Ontological Argument in The Ethics. The first thing to get clear about though is what you are supposing the term "God" to stand for - it's pretty clear that Spinoza and Descartes had different things in mind.

a variation of the Ontological Argument in The Ethics

Ethics deals with Right and Wrong usually. Why are the ontological arguments related to Ethics?

I'm not sure why Spinoza chose the title "The Ethics" for his work, but it is above all a work of metaphysics. As far as I understand him, though, he believes that his metaphysical system actually entails ethical conclusions, and I think he was of the Socratic position that all philosophy was ultimately aimed at answering one question: how ought one to live?

I'm not sure why Spinoza chose the title "The Ethics" for his work, but it is above all a work of metaphysics. As far as I understand him, though, he believes that his metaphysical system actually entails ethical conclusions, and I think he was of the Socratic position that all philosophy was ultimately aimed at answering one question: how ought one to live? — jkg20

How one should live? and if God exists? What's the relation?

Oh, hey. That is a "who" there. Whoops! Sorry. I read "Would", for whatever reason.

Woke Kantians understand that the thing-in-itself is just another name for God.

Woke Kantians understand that the thing-in-itself is just another name for God. — Thorongil

Does he think that it should be worshiped?

How one should live? and if God exists? What's the relation?

Well, and this is definitely not Spinoza's position, if you thought you could prove that the Abrahamic God exists exactly as described in the Torah and New Testaments, then the answer to the question "How should one live?" would be answered by appealing to the laws handed down directly from that god to human beings via Moses. So that would be one possible link between the questions "Is there a God?" and "How should one live?". Spinoza had a different conception of what God was, but he seems to think also that certain ways of living are to be preferred over others because they align more with the nature of the God that he supposed he had proved the existence of. I cannot possibly do justice to Spinoza's Ethics in a mere philosophy forum, and ultimately you'll have to answer for yourself the question "What, if any, moral implications follow from particular metaphysical systems?"

As far as I understand it - for Kant, the "thing in itself" is definitely not to be equated with God. However, I suppose there may be Kantians who might try to argue for such an identity. As I understand Kant, his suggestion is that you cannot reach any conclusions about God using theoretical reason because theoretical reason is confined to operate within the bounds of space and time, and whatever God might be, it is not constrained by space and time. This is also true of the thing in itself, for Kant, so I guess there is some motivation for trying to tie the two ideas together, but Kant never did so himself (as far as I'm aware). For Kant, God and religion and morality and so on become the domain not of theoretical reason but what he called "practical" reason, where the demand for intellectual rigour is a lot less strict, and you don't have to give proofs for what you believe.

Well, and this is definitely not Spinoza's position, if you thought you could prove that the Abrahamic God exists exactly as described in the Torah and New Testaments, then the answer to the question "How should one live?" would be answered by appealing to the laws handed down directly from that god to human beings via Moses. So that would be one possible link between the questions "Is there a God?" and "How should one live?". Spinoza had a different conception of what God was, but he seems to think also that certain ways of living are to be preferred over others because they align more with the nature of the God that he supposed he had proved the existence of. I cannot possibly do justice to Spinoza's Ethics in a mere philosophy forum, and ultimately you'll have to answer for yourself the question "What, if any, moral implications follow from particular metaphysical systems?" — jkg20

How should we live = Doesn't this also trigger problem of Free will? Are we supposed to have freewill? Does Christian God allow us freewill? Or which God allows us freewill? Isn't everything mapped out for us?

I think that Kant in his mature (i.e. post-Critiques) thought came to the view that the very notion of a 'proof' with respect to God was something of a conceit. After all, if it were so simple a matter that a paper could be published on it, or an experiment performed, which purported to show that 'God is real'.....I mean, the mind boggles. I think a central point of a theistic faith is the unknowability of the Divine. So to declare the Divine Nature something which can be 'proven' by logical or empirical means is, in a sense, impudent. And I think Kant had something like that view.

Interesting note from the neo-Thomist perspective provided by Jacques Maritain:

Philosophical reflection on being begins with the intuition of being, and Maritain insists that one needs this "eidetic" intuition for any genuine metaphysical knowledge to be possible. The intuition of being that lies at the root of metaphysical enquiry is not "the vague being of common sense" (see Preface to Metaphysics, p. 78), but an "intellectual intuition" (Existence and the Existent, p. 28) or grasp of "the act of existing."

This intuition of being "is a perception direct and immediate … . It is a very simple sight, superior to any discursive reasoning or demonstration [… of] a reality which it touches and which takes hold of it" (Preface to Metaphysics, pp. 50-51); it is, Maritain says, an awareness of the reality of one's being — one which is decisive and has a dominant character. This view of intuition is not, then, that of a hunch or quick insight; neither is it (Maritain continues) the same as Bergson's. It has an intellectual character, is a grasp of something that is intelligible, and requires "a certain level of intellectual spirituality" (ibid., p. 49)*. Interestingly, Maritain claims that this intuition of being is something which escaped Kant (ibid., p. 48) and many subsequent philosophers until, perhaps, the arrival of the existentialists 1 .

* I think the word that is not used here, but could have been, is 'noetic'.

I think it was the very absence of this vivid 'intuition of being' which caused Kant to declare metaphysics 'a stormy sea with no shores or lighthouses' i.e. nothing to get his bearings by. He came close to having that insight, but never broke through to it; that is my hunch, anyway.

Also see http://www.iep.utm.edu/kant-rel/#H2

Even Kant in the Critique of Pure Reason thought proofs of God were a conceit - he specifically argues that it is impossible to prove that God exists in that work. His basic argument was (and here I'm shamelessly copy/pasting from https://metaphysicsnow.com/2018/03/18/god/)

...the structure of Kant’s argument is very simple:

No argument for the existence of God will work unless the Ontological Argument works.
The Ontological Argument does not work.

Therefore, no argument for the existence of God will work.

Of course, Kant may not have been right about that.
As for intuitions of being, if "being" here is supposed to be something that comes before or transcends space and time themselves, then Kant is just going to deny that humans can have such any intuition because (at least as he was using the term "intuition") space and time are the very forms under which human beings can have any intuitions at all.

The existence of free will, and its compatibility or otherwise with causal determinism, is a philosophical mine field, which Kant also tried to negotiate in the Critique of Pure Reason. Some have argued that no moral notions make sense unless we have a very strong form of free will, and the fact that the universe and everything in it runs according to causal laws (even if those laws are stochastic) entails that we do not have that kind of free will. Therefore no moral notions make sense. Kant's basic thought about this, I think, was that arguments about causal determinism have bite for theoretical ('pure') reason, but morality is the domain of practical reason, so is untouched by any such concerns. Seems a little flimsy when stated so bluntly, and you should really read Kant for yourself as I may not be doing him justice.

Woke Kantians understand that the thing-in-itself is just another name for God. — Thorongil

Really. :meh: How would you know?