Monism in general implies the belief that the Universe is not made up of diametrically opposite concepts or entities, but just one thing that can have many manifestations but in its essential nature, remains the same. While idealistic monism argues that all is made up the mind or ideas, materialistic monism states that only the physical realm makes up everything, and the mental can be ultimately reduced to the physical as well.
Plato argued that there exists a realm of ideas where there are forms of ideas that the Universe commonly reflects, and that these immutable ideas are superior and immortal as opposed to the transient objects themselves. Thus, he argued the idea of something is more real than the thing itself. He also said that the mind is identical with the soul, but that the soul pre-exists and survives the body. Later, Berkeley and Kant added to the discourse of idealism by agreeing that all the manifestations of all that happens in the physical world have its root in the mind and in the realm of ideas.
The dualists argument of all reality being divided between matter and mind seems perhaps the most natural, at least at first glance, because in the physical world the distinctions between the mind and matter are so clearly distinguishable. Human beings tend to see the two as separate entities and see how one might function even as the other is unable to, in case of illness or injuries. Also, through the ages, folklore and religion have always made a clear distinction between two forces that govern the workings of the universe: good and evil, benevolent and malevolent, mortal and eternal.
Thus, it is easy to understand why these arguments may be considered legitimate. However, dualism talks about a very extreme universe where there dont seem to be any grey areas that merge these extreme concepts of benevolence and malevolence or body and mind. Monism attributes oneness in the entire Universe. The proponents of monism have included Parmenides, Melissus, and Spinoza and more recently, Horgan and Potrc.
They hold that there really is just one thing that makes everything up, and that it is extremely complex in the number of variations it can take. Idealistic monism stands for the concept that this one thing is the consciousness or the realm of the mind, from where everything else is conceived and thus given physical shape. If the mind had not thought up something, it would not exist in either the mental or the physical plane. Thus, whatever is physical can be brought fundamentally to the mental realm and said to have originated from there.
At the most basic level, therefore, all reality for the idealist monist originates in the mind in the form of an idea, and this idea is more real than any physical structures that it might give birth to, because it is eternal and cannot ever die or cease to be. In this way, ideas form the entire universe and will continue to do so as generations of humans live and die. I think that idealism is more plausible than dualism. Firstly, I do not agree with the dualist extremes that are always incompatible and as a sum of opposites that make up the universe.
I do not think the universe can be explained as simply as that, because at many points these two extremes do seem to converge and coexist in a way so as not to seem wholly incompatible. It does seem more plausible to me that the realm of the mind is more real than anything else because ideas are eternal and make up the world. Thus, I tend to agree with the monist viewpoint about a world that is complex but is made up of the realm of ideas that are immutable. Monistic philosophy seems to embrace the existence of seemingly contradictory things in a way that is much more accepting and broader than the dualistic view.