According to a friend, anything that exists is a being. A rock, a rainbow, a dream, an idea. I am ready to buy into the first three, but the last is difficult for me.
Is he correct?

I think this question is conflating a few different levels of analysis.

1. 'Being' in the sense of sein or dasein, which is usually taken to mean the peculiar nature of life (or human life in the latter case): i.e., very roughly equivalent to conscious experience.


2. 'Being' in the ontological sense of material existence: i.e., it has physical form, therefore it exists.


3. 'Being' in the cognitive sense that it has a functional representation in the human mind and human language: i.e., we can imagine it an talk about it, therefore it has a certain 'life' as a concept.

You can think of this in Sartre's terms as être-pour-soi (being-for-itself), être-en-soi (being-in-itself), and l'être-pour-autrui (being-for-others), though that misses some nuances (e.g., a materialist might claim that a rock only has being-in-itself and being-for others, while a spiritualist might assert that a rock also has being-for-itself in the sense of a universal consciousness). But when we talk about something like an 'idea' or a 'mood' or even a 'color'... These things clearly have being-for-others, since we can talk about them and use them for functional purposes. The question seems to hinge on whether they have being-in-themselves in some non-trivially-reductionist manner. I mean, colors have a physical reference in electromagnetic waves; moods and ideas also have physical referents in the electrochemical state of the physical brain. But it's not at all clear that there is any simple relationship between the objective states and the subjective experience.

We still have a long way to go on this issue...

Welcome to SE Philosophy!

From (SEP: Existence):

We can trace the issue of whether existence is a property to a disagreement between the ancient Greek philosopher Aristotle and some of his medieval followers over the relationship between an individual's essence and its existence.

The question you pose goes back to at least Aristotle, and is metaphysical in regards to the idea of 'being' being a category, that is to say, the statement 'X is a being' (Being(idea) = True) is a predicated proposition. Thus, the question of what belongs in the category of being would normally rely on it's properties.The question of what is the nature of the property of existence is controversial among metaphysicians since the question of what is a property is philosophically complicated because it relates to other fundamental ontological notions like relations and categories and debates over sub-classes of properties such as essential and accidental properties.

Aristotle denied that existence is actually a property, and was followed by other great thinkers (ibid.):

There is a long and distinguished line of philosophers, including David Hume, Immanuel Kant, Gottlob Frege, and Bertrand Russell, who followed Aristotle in denying that existence is a property of individuals, even as they rejected other aspects of Aristotle's views.

Starting with Aristotle, essence and existence roughly echoed form and substance, but in the modern interpretation, mind and material are generally used echoing the Cartesian duality and can be framed whether you practice phenomenology or analytical philosophy or are realist or not.

The easy answer is there is no easy answer, and depends on your metaphysical presuppositions which is why we still argue over these things.

(See strongly related SE post here.)

Surely, Ideas=Thoughts, Ideas are part of Consciousness. In the world we live in, and in our normal state, Consciousness is composed of: 1-perceptions 2-thoughts 3- feelings. Hence, Ideas are part of Consciousness. Look: https://en.wikipedia.org/wiki/Consciousness

We can generate Consciousness, i.e: we can generate thoughs, feelings, or may thoughts, feelings are imposed on us, for example: dreams, obcessive compulsive disorder.

So, dreams are kind of Consciousness, somehow a kind of wakefulness. Dreams usually are unorganized and incoherent.
What if we are, or you, alone, are in a big, ultra-real, coherent, clear, organized, orderly and regular dream. Then are rainbow, rocks are beings?, while thoughts are non beings?!. Or rainbow, rocks, thoughts are all Ideas. Then, all reality just Ideas passing or flowing in a substanse or essence we call it Mind. Thus all reality are beings, things, ideas flowing in that substance (mind)=mental existence.

If Ideas are non being, then non being entered me,or I generated it to me, this made me respond.The response of mine is being, is thing. The response of mine is eighther thoghts (Ideas) or actions or both, ideas and actions. Ideas=non being, actions=being. Then the response of mine (being) generated (non being)+(being)?.

The question is: Is Being=Thing=Existent?. Are all things imaginable, or there are some things which are unimaginable. Are unimaginables Infinite?.

The Ontological Argument

St Anselm formulated the Ontological Argument. It is an argument for the existence of God.

It is a striking instance of an idea that compels being. I'll let you read it for yourself.

I'll just say that for the thousand years following, philosophers rebutted it.

1. That so many have tried suggests it is controversial


2. That so many of varied persuasions have seen fit to grapple suggests there is something of import

So the answer to your title question is yes

For all ideas?? I'm not prepared to concede!