The Catholic Encyclopedia once defined pantheism as "The false theory according to which God and the world are one." Newer versions thoughtfully replace "the false theory" bit with "the view according to which God and the world are one." But still declare:
"the Church has repeatedly condemned the errors of pantheism. Among the propositions censured in the Syllabus of Pius IX is that which declares: There is no supreme, all-wise and all-provident Divine Being distinct from the Universe; God is one with nature and therefore subject to change; He becomes God in man and the world; all things are God and have His substance; God is identical with the world, spirit with matter, necessity with freedom, truth with falsity, good with evil, justice with injustice.
Theistic recoilment from pantheism is well recorded. Saint Augustine voiced especially vociforous opposition to the conception of pantheism, outraged to suppose that "nothing at all remains which is not a part of God" and concluding that "if this is so, who cannot see what impious and irreligious consequences follow, such as that whatever one may trample, he must trample a part of God, and in slaying any living creature, a part of God must be slaughtered?" Another Augustinean writing bemoans, "what more unhappy belief can be entertained than that a part of God is whipped when a boy is whipped? And who, unless he is quite mad, could bear the thought that parts of God can become lascivious, iniquitous, impious, and altogether damnable? In brief, why is God angry at those who do not worship Him, since these offenders are parts of Himself?"
Science and Belief
But then, human knowledge, expressed in both information science and physics, has overtaken the geocentric conceits of popes and saints. Pantheism is indeed the belief that everything is God, and God is everything, and when it first came about this was just a pretty theory -- but modern physics has taught us that it's more of a necessity than a theory. The writers of those pre-enlightenment works (and here I mean enlightenment in the general sense, and not the particular historic period) lacked the benefit of the science behind, for example, Duke University physicist Robert G. Brown's "Pandeist Theorem," published only last year (2009) -- which uses information theory to prove an "omniscient" God to necessarily be identical to the Universe anyway.
In fact, some philosophers had reached this conclusion without knowledge of physics years before, for it follows that if there is a deity which is omniscient (even relatively omniscient, such omniscience limited to interactions occurring within our limited physical Universe), such deity must have instant and perfect knowledge of all things which occur within our Universe. Consider: where is God? Everywhere, right? God as traditionally defined is pretty universally understood to be omnipresent and omniscient, meaning not only that there's no place that God is unable be at any moment, but that there can exist no place, no subatomic particle, no trasfer of any amount of energy, no thought, no act -- from which such an entity is absent. And this includes the things of which Augustine complains -- being trampled upon, whipped, slaughtered; engaging in lasciviousness, iniquity, and all manner of hedonism, debauchery, and degredation. There is no escaping this conclusion while the deity is held to be omniscient, for how can any presence or any knowledge be denied to such a being?
Consider the nature of matter and energy. All matter is itself just energy. It sounds strange, but the cells that make up your body are just made of molecules, and those molecules are made of atoms, and those atoms are made of electrons jumping around in a "shell" around protons and neutrons. This is not conjecture, powerful microscopes have actually been able to film an electron in motion!! To be clear, the electron doesn't actually "orbit" the proton, but just jumps around it like a Mexican jumping bean, but jumping across extremely tiny distances while going close to the speed of light. And what is the electron made of? Well.... nothing!! It's just a particle of electricity -- and so is the proton, with the two of them being drawn together like magnets of opposite polarity, but also repelling each other (through a different force) like magnets of identical polarity. That's why the electron stays inside the "shell" -- by the way, there's not a real physical shell, just a distance that the electron won't go beyond unless it's pulled away by another proton. And that is how molecules form, because an electron is jumping back and forth between the shells of two or more atoms, tying those atoms together.
Now, where is God in all this? Well, what is sustaining those electrons, keeping that charge in existence? That, the pantheist must argue, is the constant sustainment of God. In every electron and proton and neutron in our entire Universe. Just like a dance exists only in someone's mind until a dancer performs it, and then the dancer is the dance until she stops dancing, God is the electrons until God stops sustaining them. And God can not stop sustaining them (at least, not on a whim) because God is consistent. It's as simple as that.
So, is man God? You are made of atoms, made of subatomic particles, every single one of which is just an idea of God's perpetually sustained by the will of God. Your fingers and toes are made of these atoms. Your brain and all your nerve cells are made of these atoms. When you think a thought, is it possible for God to not know it? If you stub your toe, is it possible for God not to feel exactly the pain you feel? (Well of course not, if it was then you'd know something that God didn't -- not very likely, is it?) And every time some military force drops a bomb on a village and kills an innocent bystander, God is seared with an eternal perfect knowledge of all the pain of that person being killed, and all the anguish of everyone who cared for that person facing their loss. There's no way for that to be avoided so long as we recognize God as "all-knowing."
But within this broad and malleable pantheistic frame, the range of thought extends from ecstatic nature worship, to complementarily denial of any form of divinity whilst calling the physical Universe "God" (which is what lead Richard Dawkins to label pantheism as "sexed-up atheism"), to an odd form of nihilism which preaches that the state of human awareness is nothing but a distraction from a higher state of universal awareness experienced by non-living things, inviting suicide and hailing death itself as a liberation from the bondage of the physical human mind. The nihilistic pantheists, believing "dead" matter to be more aware than live, tend to be equally hostile to theisms and to theological formulations traditionally less noxious to pantheism, such as Buddhism and Pandeism.
Pantheism, in any sense, denies at least one, if not both, central theistic claims: the "personal" nature oft attributed to the the divine, and the "transcendant" nature of the same. Pantheists deny the "otherness" of the deity, and the ontological distinguishability of the world from the divine. Lacking specific doctrines or views on creation, what may positively be claimed of pantheism is its set of possibilities passed upon by theism, the latter being tied down to scriptural extrapolations.
Pantheism continues, as well, to connotate the rejection of theism. A modern pantheist organization tells us: We use the name pantheism because it has a long and venerable history. But our beliefs are entirely naturalistic, and compatible with atheism, humanism and naturalism. Also with those forms of paganism that see magic and the gods as symbols rather than realities. We offer a home to all forms of naturalistic spirituality - scientific pantheism, religious humanism, religious naturalism, positive atheism, deep ecology, philosophical Taoism, modern Stoicism, Gaia religion, also Western forms of Buddhism that celebrate Nature and everyday life, and to those in Unitarian Universalism who do not believe in supernatural beings. You are completely free to adopt the terms and practices you prefer. Most of us avoid "god-language" and the sizeable minority who use it do so metaphorically.
An historical aside
The term originated with John Toland in his "Socinianism truly Stated" and with his opponent Fay in "Defensio Religionis" (1709). Toland published his broadest statement on the doctrine, the "Pantheisticon" in 1732. The theory gained wide acceptance among the philosophical classes of the late 18th and early 19th centuries, so much so that Arthur Schopenhauer wrote: On the whole, one might be surprised that even in the seventeenth century pantheism did not gain a complete victory over theism; for the most original, finest, and most thorough European expositions of it (none of them, of course, will bear comparison with the Upanishads of the Vedas) all came to light at that period, namely through Bruno, Malebranche, Spinoza, and Scotus Erigena. After Scotus Erigena had been lost and forgotten for many centuries, he was again discovered at Oxford and in 1681, thus four years after Spinoza's death, his work first saw the light in print. This seems to prove that the insight of individuals cannot make itself felt so long as the spirit of the age is not ripe to receive it. On the other hand, in our day (1851) pantheism, although presented only in Schelling's eclectic and confused revival thereof, has become the dominant mode of thought of scholars and even of educated people. This is because Kant had preceded it with his overthrow of theistic dogmatism and had cleared the way for it, whereby the spirit of the age was ready for it, just as a ploughed field is ready for the seed.A Europe-wide controversy (the Pantheism controversy) erupted in the 1780s, when Friedrich Heinrich Jacobi equated pantheism, as supposedly set forth in the works of Baruch Spinoza (who never described himself as a pantheist, though many pantheists have adopted certain of his writings as supportive of pantheism) with materialism -- which, in that day, was no different from calling it atheism. Thinkers lining up on the other side of the issue included Moses Mendelssohn and Immanuel Kant, who denounced Jacobi for supposedly abandoning reasoned approaches to the divine.
For the great length of its existence, the arguments raised against pantheists by theists were, surprisingly, scientific -- the conception for example that the fossil record and indicators of the evolution of civilization, showed that the Earth had a point of origin. Pantheists contended against these arguments with the naked assertion that the world, the Universe, had simply always existed, as had mankind, and ours was simply a peak of civilization echoing many which must have come before, and then subsided. Indeed, even in the Twentieth Century, proponents of Edwin Hubble's Big Bang theory tended to be religious, favoring that theory in part for its assignment of a point of origin to the Universe which accorded with Creation mythology. The scientific opposition to the Big Bang was lead by Fred Hoyle, whose steady state theory proposed that instead of having a point of origin, the Universe was eternal, and had constantly been suffused with new matter. And while proponents of this view tended to be atheistic instead of pantheistic, this was, as well, the view which accorded with pantheism.
And so, there remains a spectrum of origin-beliefs, with certain Creationist theists proposing that our Universe came into existence just within the past few thousand years, scientists and certain theists following the Big Bang model and evolutionary model to hold our Universe to be around 13.728 billion years old, and some (by no means all) pantheists who continue to hold that there never was a beginning, perhaps invoking an oscillating Universe or steady-state-of-Universe-generation model to discount the current science.
Objections and Derivations
A number of issues have been raised with pantheism, including the above theistic objections to the idea of a "God" sharing in experiences which we consider to be indignaties, and both theistic and scientific objections to the idea of a Universe which has no point of origin. Theists also have faulted pantheism for its lack of explanation for the existence of scripture and the reports of miraculous interventions, though these have been answered by pointing to human origins for these conceits, rather than divine. But the greater failing accorded to pantheism is not that it equates all things with God (as all things must be, at the least, either of or within such an entity if it exists at all), but that it offers no explanation of how or why anything has come to exist at all, and especially why this particular Universe has come about, nor of its purpose. And so, later combinatorial efforts have sought to close the circle on these concerns, commending the wisdom of its successes but attempting to otherwise bring it into accord with scientific and philosophical advances.
The same Catholic Encyclopedia noted above provides nary a word on the slightly unbalanced twin offspring of pantheism, big brother panentheism and little brother pandeism. It has actually been wondered if, in the ruminations of the ancients of India and Greece (whose traditions we so often dialectically misuse), if conceptions of panentheism and pandeism did not precede the concept now denoted as pantheism. This possibility is forecast by English occultist Godfrey Higgins, who picked up this line in the 1830s, briefly supposing:
"Many persons have thought that this Pan related to what has been called Pantheism, or the adoration of universal nature, and that Pantheism was the first system of man. For this opinion I cannot see a shadow of foundation. As I have formerly said, it seems to me contrary to common sense to believe that the ignorant half savage would first worship the ground he treads upon,--that he would raise his mind to so abstruse and so improbable a doctrine as, that the earth he treads upon created him and created itself: for Pantheism instantly comes to this."
But panentheism and pandeism are not properly called subsets of pantheism, for in both are elements initially rejected by pantheism. Each, instead, simply incorporate those elements of pantheism which comport with logic and physics into a broader frame -- panentheism folds pantheism back into theism to suppose that, yes, the divine IS our Universe, but that the Universe was created from some part of itself, while another part of it transcends our Universe as well. Panentheism later gave rise to panendeism, which supposes the transcendant aspect of the divine to nonetheless be inactive in human affairs. And, finally, Pandeism seeks to combine pantheism with the insights of deism to find that, yes, the deity is currently our Universe, but that it was as well the Creator of this Universe, creating by becoming in order to share fully and without distraction in the experience of this existence.