Ideas Indivisible: the similarity between Democritus’ atoms and Plato’s Forms
For the sake of simplicity, this article will treat all the content in the Platonic writings as the words and ideas of Plato. Similarly, when writing of Democritus, I speak also of those doctrines inherited by him from his teacher Leucippus.
“Aristoxenus in his Historical Notes affirms that Plato wished to burn all the writings of Democritus that he could collect.” 
This extract from Diogenes Laertius, recording Plato’s desire to burn the work of his near-contemporary and fellow philosopher, Democritus, reveals a somewhat incongruous episode in the Athenian’s life. The show of outward animosity, rather unexpected from that same philosopher who dismissed the poetry of Homer from his ideal city only reluctantly, and with a loving regret, indicates a powerful enmity between the two great thinkers. For that reason, their juxtaposition in this article feels rather like that clash-of-the-Titans style confrontation used recently to advertise the presidential election debates in the US; and, moreover, seems just as likely to culminate with a Platonic Biden — or rather, a Bidenian Plato — resorting to the frustrated exclamation of, “Will you shut up man?” as his notoriously cheerful opponent laughs on.
Yet, at a closer glance, the two philosophers appear to have certain ideas and attitudes in common. Is not Democritus’ declaration that “In reality we know nothing,” reminiscent of Plato’s own paradoxical construction, “I know that I know nothing”? Furthermore, like Plato, Democritus privileges intellect over the senses in the pursuit of knowledge when he remarks that the senses lead to illegitimate (or broad) judgement, while the intellect produces legitimate, more precise judgement. In this article, I will briefly explore the idea that the philosophies of Plato and Democritus are not utterly opposed to one another, but rather share common ground, by examining two of their best known doctrines — Plato’s concept of Forms, and Democritus’ concept of atoms. The article aims to show that, although these concepts diverge in many places, both of them have their basis in the same philosophical issue — the search for stable identity — and both tackle this issue in much the same way.
The problem of stable identity is interwoven through the doctrines of PreSocratic philosophy. Heraclitus’ belief that ‘everything flows’, expounded in the hypothesis that ‘a person cannot step in the same river twice’, demonstrates a concern about the constancy of change and the mutability of identity. In contrast to this, but reacting no less to the same problems, Parmenides theorised that change was impossible and identity therefore fixed (a belief supported by those paradoxes of the Eleatic Zeno). The ideas of both Heraclitus and Parmenides reveal that problems of temporal change and its impact on identity troubled — and excited — philosophers in the 5th century Greek world. It was into this context, into these questions, that Plato and Democritus advanced with their respective concepts of Forms and atoms; and both met the problem with solutions that were fundamentally similar: the existence of indivisible, unchanging constituents, which generated all mutable objects and allowed for the existence of absolute identity.
That atoms are indivisible is evident from their name alone. The adjective ἄτομον (transliterated into the Latin alphabet as átomon, and hence the English ‘atom’) describes something uncut, or indivisible. Moreover, when referencing the doctrine of Leucippus and Democritus, Aristotle wrote of their belief in the “indivisible bodies”, of which all things are composed. In a similar way, Plato presents Forms as indivisible entities. The philosopher’s belief that Forms are indivisible is demonstrated by the very notion of what a Form is. In describing these entities, Plato lays an emphasis on the singularity of their nature. In Book X of the Republic, the philosopher writes that there cannot be two Forms of ‘Bed’, for if there were two beds, “a single one would be discovered beyond them whose Form both of these beds would share, and that would be the actual bed, not these two.” Again, when he writes of classing material objects and characteristics, “in terms of a single Form of each of them on the grounds of there being one real one in each case,” he demonstrates that the forms are unique in their nature. This clear belief in a Form holding one singular and unreplicated identity reveals the significance of that identity to the very existence of the Form. A Form is only a Form because of the uniqueness of its identity; and so, if that unique identity ceases to exist, the Form will cease to exist. Hence, a Form cannot be divided: even if the ‘Form of Bed’ could somehow be torn apart — by the formal versions of the Bacchae perhaps, or by a formal handsaw — it would then cease to be the Form, as it would not have the same qualities as before the division (its size and proportions would have changed) and thus would no longer possess the unique identity of ‘Bed’. Hence it can be seen that both atoms and Forms are indivisible entities.
Along with being indivisible, atoms and Forms are also unchanging in themselves. That is to say that their properties (shape, texture, etc.) do not and cannot change. According to Aristotle, the atomists Democritus and Leucippus theorised that atoms are the cause, rather than the subject of change, that alteration exists as a result of their movements and interactions. This belief in ‘changeless changers’ answers the PreSocratic question of whether alteration is possible with the ever-philosophical response of ‘well yes, but actually no’. Atoms allow for both an explanation of the change which humans perceive around them, and, simultaneously, a more fundamental changelessness, offering a refuge for stable identity in what appears to be a constantly morphing world.
Forms too are characterised as unchanging. For Plato, the material world is constantly altering itself: the natures of objects and characteristics were transformed by time, position, relation, and respect. This belief is evident in the Republic, when (following a prompt from Socrates) he has Glaucon admit that those material objects which one might consider beautiful, “must somehow appear to be both beautiful and ugly, and similarly with the rest that you asked about.” This indicates that the nature of objects in the material world are subject to change. Likewise, in the Phaedo, Plato writes about the characteristic of equality in two material objects, asking whether, “equal stones and pieces of wood, even though they are the same ones, seem equal to one person but not to another?” This shows an awareness that the qualities perceived in material objects change on the basis of by whom they are perceived. And again, in the Timaeus, Plato speaks of, “the conditions which Becoming has attached to the things which move in the world of Sense.” The conditions of becoming (an example of which he gives as growing old) are set in opposition with the conditions of being, which he attributes to the immortals. The distinction between the world of the senses “becoming” and the immortal world “being” (or existing) illustrates the instability of identity in the material realm. Later, Plato expands on this idea through the analogy of a figure of gold which was constantly being remoulded. When asked what it is, the figure cannot offer a response before it is again altered, a statement reminiscent (or rather, anticipant) of that ontological cry, “I cannot be for all this interminable becoming!” Through this analogy, Plato demonstrates the disposition of the material world to constant change: nature is unstable, no identity is fixed.
Forms are presented as a solution to this fluctuation: unchanging essences. For Plato, the Forms exist unaffected by the transformations which act on the material world. He shows, for example, that the identity of the Form is unaltered by time, when he writes that, “As essence is to generation, so is intellection to opinion.” Elsewhere in the Republic, Plato teaches that Forms exist in the realm of intellect, while material objects and characteristics are found in the area of opinion. Therefore, in relating essence to generation as intellect is related to opinion, the philosopher connects essence — constant existence, that is — with the Forms, and opposes this state to the instability of the material world. Similarly, in the Phaedo, Plato asks whether Absolute Equality, Absolute Beauty, or any other existence absolute allows any kind of change; to which his companion’s response is that such an essence, “must always keep its identical state.” Hence it is made clear: Forms are unchanging.
One might argue against this equation of atoms and Forms, on the basis that one concept is physical, while the other is intellectual. While it is true that Plato’s Forms are abstracted from the material world and Democritus’ atoms are ingrained in it, nevertheless, both entities are ultimately conceptual and removed from the realm of human perception in a similar way. Plato writes of his Forms being invisible to humans, imperceptible through sensation and only accessible via the intellect. Atoms too are invisible to human perception, due to their size. Because of this, they are not accessed through sense perception, but instead are found, like the Forms, through intellect. Moreover, although in our post-Enlightenment society it is all too easy to think of atoms as the product of empiricism — and therefore more grounded in physical existence than the abstract philosophical concept of Forms — it is worth remembering that Democritus’ belief in atoms came to exist through a process of reasoning and judgement, and not through scientific experiment. Furthermore, forms and atoms contribute to and interact with the world of perception in similar ways. Just as the characteristics of atoms provide the characteristics of those perceptible objects which they create, Plato’s Form’s offer outlines for our human perceptions of objects. This is clear from Plato’s example of the ‘Form of Bed’ and the many (material) beds. In writing that these material beds share the form of the essential Bed, Plato indicates that the characteristics of Forms determine the characteristics of objects and qualities in the material world. Thus, both Forms and atoms interact with the sensory world, but are removed from this plain of existence. Fundamentally, both entities exist outside the realm of human perception.
This article does not aim to deny that there are significant differences between Plato’s Forms and Democritus’ atoms. Many aspects of the theories distinguish them from one another. Forms, for example, can exist as properties (Beauty, Justice, Temperance), while atoms only possess properties, without themselves signifying those characteristics. Again, while Forms translate their images directly (though imperfectly) onto the objects of the material world, the products of atoms do not directly resemble those atoms. In spite of these differences, however, the similarities between Forms and atoms nevertheless demonstrate their shared origin in the search for stable, unalterable identity.
As humans, we are a species of words — we attempt to tame our world through language, confine it to titles, and yoke it with the bit of terminology. Our existence is built on this. Indeed, Plato himself obsessed over definitions, seeking to understand the universe through what could be clearly and categorically spoken. Through terms like ‘justice’, ‘beauty’, and ‘love’. We want to view the objects of this world as clear-cut, recitable, and ultimately identifiable. Amongst this species of animal, fixated as it is on terminology, it is no wonder that philosophers have found such difficulties in the mutability of life, no wonder that they have been dismayed at the universe’s refusal to stay still long enough that it might be called ‘the universe’, with a proper understanding of what that term means. Both atoms and Forms provide a solution to these issues of identity — indivisible, unchanging entities, which offer some constancy beyond the impurities of the perceivable world. Where our senses experience change and obscurity, Democritus’ atoms and Plato’s Forms provide the mind with the absolute identities which it seeks.
 Diog. Laert. IX.40.
 Pl. Resp. X.595cff.
 Diog. Laert. IX.72.
 Pl. Ap. 21d.
 Sext. Emp. Math. VII.139.
 Nussbaum & Schofield, 2016.
 Palmer, 2019.
 Arist., Gen. corr. I.i.314a.
 Pl. Resp. X.597c.
 Idem. VI.507b.
 Arist., Gen. corr. I.ii.315b.
 Vlastos, 1965 p14.
 Pl. Resp. V.479b.
 Pl. Phd. 74b.
 White, 1992 p.280.
 Pl. Ti. 38a-b.
 See White, 1992 for a discussion on this.
 Pl. Ti. 50a-b.
 Pl. Resp. VII.534a.
 E.g. Pl. Resp. VI.508e.
 Pl. Phd. 78d.
 Idem. 79a; Resp. 509dff.
 Arist., Gen. corr. I.viii.325a.
 Pl. Resp. X.597c.
Ancient Sources ~
Aristotle, On Coming-to-be and Passing Away, tr. E. S. Forster, D. J. Furley, Loeb Classical Library 400, Harvard University Press, (Cambridge, MA, 1955).
Diogenes Laertius, Lives of Eminent Philosophers, Volume II: Books 6–10, tr. R. D. Hicks. Loeb Classical Library 185, Harvard University Press, (Cambridge, MA, 1925).
Plato, Apology, tr. C. Emlyn-Jones, W. Preddy, Loeb Classical Library 36, Harvard University Press, (Cambridge, MA, 2017).
Plato, Phaedo, tr. C. Emlyn-Jones, W. Preddy, Loeb Classical Library 36, Harvard University Press, (Cambridge, MA, 2017).
Plato, Republic, Volume I: Books 1–5, tr. C. Emlyn-Jones, W. Preddy. Loeb Classical Library 237, Harvard University Press, (Cambridge, MA, 2013).
Plato, Republic, Volume II: Books 6–10, tr. C. Emlyn-Jones, W. Preddy. Loeb Classical Library 276, Harvard University Press, (Cambridge, MA, 2013).
Plato. Timaeus, tr. R. G. Bury, Loeb Classical Library 234, Harvard University Press, (Cambridge, MA, 1929).
Sextus Empiricus, Against Logicians, tr. R. G. Bury. Loeb Classical Library 291, Harvard University Press, (Cambridge, MA, 1935).
Secondary Literature ~
Nussbaum, M., & Schofield, M., “Heraclitus (1), son of Bloson of Ephesus, fl. c. 500 BCE,” in The Oxford Classical Dictionary, (2016).
Palmer, J., “Parmenides of Elea, Presocratic philosopher, c. 515–post-450 BCE,” in The Oxford Classical Dictionary, (2019).
Vlastos, G., “Degrees of Reality in Plato,” in New Essays on Plato and Aristotle, Renford Bambrough (ed.) (London, 1965), 1–19.
White, N. P., “Plato’s metaphysical epistemology,” in The Cambridge Companion to Plato, by R. Kraut (ed.), Cambridge University Press, (Cambridge, 1992) pp.277–310.