'Monad' literally means 'unit'. In its broadest sense, it can therefore refer to any natural system or entity, any unitary individual -- from the vastest solar system to the tiniest atom. In general, every entity, on any plane, has within it a nucleus, a 'control centre' or 'nerve centre', of more highly developed energy-substance belonging to that plane, and this nucleus, too, can be called a monad. The term is commonly used in a still more restricted sense to refer only to the highest, spiritual portion of any entity's multilevelled constitution.
On our own physical level, the entire solar system can be called a solar monad, with the sun at its heart. The sun, in turn, can be called a solar monad, built up of smaller monads, with its own governing monad at its centre. The human physical body can likewise be called a monad, made up of smaller monads, with perhaps a central monad in the heart or brain. At the same time, every physical entity is animated and guided from inner, subtler levels of its constitution, and each of its more ethereal bodies or souls (or aggregated monads) likewise has its own central nucleus (or monad), with the very highest monad (or monadic essence) in overall control. The human constitution contains the following monadic centres: a divine monad, a spiritual monad, a higher human monad, a lower human monad (which we are at present), a beast monad, and an astral-physical monad. Each of these monads possesses a fund of stored-up evolutionary experience (or ego) and works through an appropriate vehicle (or soul).
The general meaning of 'monad', as used in theosophy, is defined by G. de Purucker as follows:
A spiritual entity which to us humans is indivisible; it is a divine-spiritual life-atom, but indivisible because its essential characteristic, as we humans conceive it, is homogeneity ... Monads are eternal, unitary, individual life-centers, consciousness-centers, deathless during any solar manvantara, therefore ageless, unborn, undying. [They] are spiritual-substantial entities, self-motivated, self-impelled, self-conscious, in infinitely varying degrees, the ultimate elements of the universe.
Regarding the evolution of the monad, he writes:
Every monadic essence, every monad, no matter where or in what period of time, is a learning entity, always advancing from the less to the more perfect. In any one cosmic manvantara it begins its evolutionary journey as an unself-conscious god-spark, passing through all the phases and experiences which that particular manvantara contains, and finishes as a fully perfected god.
Since a monad takes one planetary manvantara, or seven rounds, to advance one kingdom, ten reembodiments of any planetary chain are needed for a monad to advance from the lowest of the three elemental kingdoms to the highest of the three dhyani-chohanic kingdoms.
The number of individual monads in each of the four visible kingdoms on earth -- the mineral, plant, animal, and human kingdoms -- differs widely. For instance, there are some 6 billion human beings on earth, and many more billions currently in their after-death states. But in each human body alone there are some 100 trillion cells and about 10 trillion trillion trillion atoms. So obviously the individual monads dwelling in atoms (and subatomic particles) in and on the earth cannot all become individual human monads on the earth as we know it.
A solution is indicated by mahatma KH when he says that the human soul is 'a compound of the progressed entities of the lower kingdom'.8 This suggests that just as human physical bodies are built up of countless multitudes of atomic and subatomic entities, a human monad can be seen, by analogy, as a relatively integrated aggregate of what were once more highly differentiated (though from our perspective relatively unindividualized) mineral monads. This does not mean that the 'individualities' of former mineral monads are completely destroyed -- merely subsumed within a greater whole.
To view the matter from another angle: the divine monad within us, whose field of action is the galaxy, emanates many spiritual monads, each of which is active in a particular solar system; each spiritual monad emanates several higher-human or chain monads, each restricted to a particular planetary chain; and each chain monad emanates twelve lower-human or globe monads. Each parent-monad contains its children-monads within it as part of its constitution. Similarly, the monad of an atom probably encompasses the monads of all the atom's constituent particles.
Monads remain distinct individuals throughout their manifested existence in a particular evolutionary cycle or manvantara. The divine monad endures for a life-term of the galaxy, the spiritual monad for a life-term of the solar system, and the higher-human monad for a life-term of the planetary chain. At the end of its life-cycle, each monad is not destroyed but rises to higher planes of consciousness for a period of rest, until the next cycle of activity begins.
The higher qualities gained through evolution are never lost. This is exemplified by what happens to our personalities after death. At the 'second death' in the lower astral realms or kama-loka, everything of a noble and spiritual character that the soul retains is withdrawn into the upper triad (atman-buddhi-manas), or the (relatively) immortal monadic essence, of our sevenfold constitution, and the aggregate of these indrawn elements constitutes the human monad.
This illustrates the fact that all real, finite monads are composite, however relatively homogeneous and indivisible they may be from our viewpoint. Speaking generally, there are no limits to how big or small a monadic entity can be. Their range of size is infinite, bounded only by the two abstract 'limits' of the infinitely large and the infinitely small. Every infinitesimal point of space can be regarded as a monad, all of them contained within the one infinite Monad; but these 'ultimate' monads are of course abstractions and not real entities. Our own spiritual monad may be unimaginably small from our standpoint, and yet contain an infinite number of worlds within it, just as our entire observable universe may be just a tiny particle in the body of some supercosmic entity. As above, so below.
Our spiritual monad is therefore a composite entity, a 'group of monadic lives':
It is the cosmic seed out of which the tree of cosmic life grows, its roots upwards; its branches, branchlets, twigs, leaves, fruits, below. Eventually out of such a seed come all the issues of cosmic life, and if there were but one infinitely homogeneous essence in the monad, there could not be this dispersing into the manifold, incredibly numerous, innumerable planes, beings, hierarchies of the manifested universe. The very fact that heterogeneity issues from what we call homogeneity shows that heterogeneity with all its innumerable branches was locked up in the homogeneous monadic essence, streaming through that homogeneous monadic essence as the life stream of a tree issues forth from the apparently, and to us actually, homogeneous seed. ...
Thus then, the monad to us is homogeneous. All issues forth from it, all again will be reabsorbed or withdrawn back into it when pralaya opens. But all these different individualities must have been lying latent there in order that they might issue forth therefrom. Therefore it shows us that the monad is not merely a channel, a laya-center, through which streams ... whatever there is of manifested life, but that the monad itself is an entity.
De Purucker says that any atom, plant, or animal is an expression of its indwelling monad just as much as a human being is, but that the farther back we go towards the chemical elements, the more closely such monads are bound together in families.
However, in her article 'The mineral monad', Blavatsky seems to oppose the idea of individual monads for each atom or subatom, saying that 'the mineral monad is one -- the higher animal and human monads are countless'. She writes:
It would be very misleading to imagine a monad as a separate entity trailing its slow way in a distinct path through the lower kingdoms, and after an incalculable series of transmigrations flowering into a human being; in short, that the monad of a Humboldt dates back to the monad of an atom of hornblende. Instead of saying a mineral monad, the correcter phraseology in physical science which differentiates every atom, -- would of course have been to call it The Monad manifesting in that form of Prakriti [original substance] called the mineral kingdom. Each atom or molecule of ordinary scientific hypothesis is not a particle of something, animated by a psychic something, destined to blossom as a man after aeons. But it is a concrete manifestation of the Universal Energy which itself has not yet become individualized: a sequential manifestation of the one Universal Monas.
She adds that the monadic essence begins to differentiate imperceptibly in the vegetable kingdom, does so to a greater degree in the animal kingdom, and becomes distinctly individualized in the human kingdom.
Thus Blavatsky, like De Purucker, states that the degree of individuality increases as we ascend from the mineral kingdom to the human kingdom. However, Blavatsky says that the universal monadic energy manifesting through the mineral kingdom is undifferentiated on its own plane, whereas De Purucker says that the overall mineral monad is composed of individual monadic entities, but that they do not possess the distinct individualities that, say, human monads do. In other words, both writers are basically saying the same thing, but from two different angles. Indeed, elsewhere Blavatsky herself says that 'every cell in the human body has its monad, as every cell in animal, vegetable, and even in the (so-called) inorganic bodies'.16 The key to the matter is that every monad is both one and many, composed of lesser monads, and living its life as part of greater monads. Ultimately all separateness or differentiation is relative and transitory, and in that sense 'illusory'.
That there is an individual monad in every atom etc. also follows from the analogy between an atom and a solar system. From our point of view, all electrons are exactly alike in terms of their measurable properties. However, what we call an electron is probably an illusion produced by the incredibly rapid reembodiments taking place on the subatomic level. The entity (or entities) that produce the phenomenon we label 'an electron' is no doubt just as complex in its own right as our earth is. So to deny that each electron carries within it a higher monadic aspect would be equivalent to denying the same for the earth. The monad of an electron must be just as complex and individualized on its own scale of things as the planetary spirit or monad of the earth. But just as all the monads in the mineral kingdom can be thought of as one (relatively) undifferentiated, homogeneous monad from our viewpoint, so might all the planetary and solar monads in our region of the universe be considered as one monad from the viewpoint of some supercosmic entity. It's all a question of relativity and perspective.
According to De Purucker, the idea that all the entities in the mineral, plant, and animal kingdoms are part of a 'group-soul', which they merge into after death, is misleading but not altogether wrong:
If we remember that individualization proceeds steadily from the elementals up to the Dhyan- Chohans, and that all monads belong to or are affiliated with, one or other of the seven or ten or twelve Solar Logoi, or what are called 'Rays' by some, we at once see the real teaching behind this group-soul idea. ...
The monads in the lower kingdoms are much more closely interlocked and more alike each other than are the more individualized monads in the higher kingdoms. But it is entirely wrong, wholly erroneous to say that the monad of a beast, for instance, sinks back into an ocean of soul, or into a group-soul, and never again issues forth, but that merely new differentiations of the ocean issue forth, new droplets.
The monad, he says, travels through time and space constantly, struggling to express itself more fully as its vehicles become more greatly individualized, but it is an individual from its earliest appearance in any manvantara as an unselfconscious god-spark.
As monads progress through the three superhuman (or dhyani-chohanic) kingdoms, individuality (in the sense of individualism) decreases while integration increases -- the opposite of what happens in the three submineral, elemental kingdoms. Both elementals and dhyani-chohans have neither individuality nor personality as separate entities: 'Individuality is the characteristic of their respective hierarchies, not of their units; ... the nearer to the region of Homogeneity and the One Divine, the purer and the less accentuated that individuality in the Hierarchy.'18 But whereas the elementals are 'blindly and unconsciously submissive to their respective kingdom-hierarchs because they have not sufficient egoity to become intellectual rebels as men so often are', the dhyani-chohans are becoming 'more fully self-conscious divine or spiritual egos, and thus their subservience to their hierarch is a gladly willing one'.
Just as we humans are composite entities made up of hosts of beings, so is the hierarch, the supreme atman or paramatman, of any hierarchy. Although it possesses its own individuality and can be regarded in a sense as indivisible, it is the aggregate of the monadic essences of all the entities composing its hierarchy or family. It is the source of all the subordinate entities flowing from it, which it permeates with its vitality. Similarly a human being is an aggregate of life-atoms of many degrees, existing in many stages of consciousness, all permeated by the stream of consciousness issuing from atman, the inner god. This line of thought can be applied to all living entities, on any scale.
De Purucker also points out that the entities occupying the rising grades of the ladder of life become fewer and fewer as the summit of a hierarchy is reached. In a sense, the soul of each individual member of the higher kingdoms increasingly becomes a 'group-soul', i.e. an integrated combination of once more greatly differentiated monadic entities, and the distinctness of every soul decreases as relative perfection is approached. At the end of a manvantara, the many are reabsorbed into the One, i.e. the multitudes of differentiated monads are reintegrated into the ultimate 'group-soul' or over-soul -- the Parent Monad, Absolute, or Silent Watcher of the hierarchy in question. But even then they do not entirely lose their individuality and become a perfectly homogeneous mass, otherwise they would never be able to issue forth again, after the long cosmic rest-period or pralaya, to continue their evolutionary journey.
At the start of the next cycle the monads differentiate again into separate kingdoms, subkingdoms, etc., according to their evolutionary status. Referring to the first human root-race in the present fourth round on the earth-chain, for example, Blavatsky speaks of seven human groups evolving simultaneously on seven different portions of the first continent, and says that their capabilities and characteristics were very different, 'to suit the Karma of the various reincarnating Monads which could not be all of the same degree of purity in their last births in other worlds'.
It is sometimes said that humans are destined to become stars, or more specifically the spirit-souls of stars.23 But just as it is impossible for every individual mineral monad to become an individual human monad on our own planetary chain due to the vastly different numbers of these two types of monads, so it is impossible for every individual human to become a separate star on our own scale of things, since the number of humans or equivalent beings in our galaxy must exceed the number of stars by an immense factor. Even collective humanity on earth could not become a star on our own scale of things, as all mature stars are said to have a family of planets, on each of which a 'human' kingdom evolves, and therefore the number of humanities in the galaxy must greatly exceed the number of stars.
But in the boundless universe, cycles of evolutionary activity unfold on an infinite number of planes and on an infinite scale of size, and it is not size that determines evolutionary status but the development of consciousness. If every atom in our bodies is a miniature solar system, and every atomic nucleus is a miniature star, then even our own bodies contain untold multitudes of stars!