"Early Buddhism" refers to the layer of teachings that can be reconstructed as predating the split of the tradition into distinct schools — Theravāda, Sarvāstivāda, Mahāsāṅghika, Dharmaguptaka, Mahīśāsaka, and others — each of which carried forward its own version of the discourses. The four Pāli Nikāyas (Dīgha, Majjhima, Saṃyutta, and Aṅguttara) are one such school's record of these teachings. But parallel collections survive in Chinese translation as the four Āgamas (Dīrgha, Madhyama, Saṃyukta, and Ekottarika), along with scattered discourses in Sanskrit fragments and Tibetan translation. Each of these lines of transmission was carried by different reciter lineages, in different languages, for centuries, before being written down.

Anālayo makes an important clarification in the introduction to Dīrgha-āgama Studies: these collections are not simply "the product of a school." They are the end result of oral transmission across many generations of Indian reciters, of whom only the final generations can be identified with a particular school. Sectarian affiliation is real and relevant, but it does not explain every difference between versions — much of the variation is simply what happens to any text transmitted by memory over centuries. Early Buddhism, in this sense, is not one static set of scriptures but the reconstructable core that emerges when independent lines of transmission are read against one another.

The value of going back to this earliest reconstructable layer is that it lets one distinguish, with reasonable historical confidence, teachings likely to stem from the earliest period of the tradition from doctrinal elaborations that were added later — whether by Abhidharma-style systematization, sectarian dispute, or (in some collections) the early stirrings of Mahāyāna thought. Anālayo's chapter "The 'Early' Discourses" in Dīrgha-āgama Studies works through this directly: it examines commentarial additions the Pāli tradition itself acknowledges (such as verses added in Sri Lanka to the Mahāparinibbāna-sutta), later doctrinal levelling across versions, and the political and geographical setting of the discourses, in order to estimate how far back their present form can reasonably be traced.

This matters for anyone who wants to engage with Buddhist thought at its source rather than through the lens of a single later school's interpretation. Reading a discourse against its parallels also has a very practical benefit: differences between versions frequently expose copying errors, mistranslations, or later insertions in whichever version is better known — which is usually the Pāli, since it is the one most translated and taught in the West. Comparative study is therefore not just an academic exercise; it is a check on how much any single version of "the Buddha's words" can be taken at face value.

This is the methodological heart of all four volumes. Each Pāli discourse in these studies is set beside its Chinese Āgama parallel (and, where available, Sanskrit or Tibetan material), and the differences are examined discourse by discourse. Some differences are minor — a stock phrase handled differently by a translator, a list given in a different order. Others are substantial: whole episodes present in one version and absent in another, or doctrinal formulations that appear more developed in one line of transmission than in its counterpart.

Because the Pāli and Chinese lines were transmitted independently, by reciters of different schools, in different languages, agreement between them is a much stronger indicator of early, shared material than anything found only in a single version. Conversely, an idea that surfaces only in the Pāli, or only in one Chinese Āgama, is a candidate for later addition by that particular lineage. This is precisely the logic Anālayo applies — and also the logic he tests critically, engaging directly with scholarly counter-arguments (Gregory Schopen's proposal that agreement across versions can sometimes indicate a later "levelling" rather than an early core is examined at length in Dīrgha-āgama Studies). The comparative method, in other words, is not a mechanical rule but a historical argument that has to be made and defended case by case.

None of the four Āgama collections is the property of only one school in any simple sense, but each does show identifiable sectarian fingerprints. The Madhyama-āgama is generally attributed to a Sarvāstivāda line of transmission (a question Anālayo examines in detail in its appendix, alongside a competing view that questions this attribution). The Saṃyukta-āgama translated by Guṇabhadra reflects a Mūlasarvāstivāda original, apparently obtained in Sri Lanka. The Dīrgha-āgama shows signs of Dharmaguptaka transmission. The Ekottarika-āgama's school affiliation remains genuinely unsettled — and this collection is also the most doctrinally "advanced" of the four, incorporating material that reflects the emerging bodhisattva ideal and explicit Mahāyāna/Hīnayāna terminology.

This is why the three appendices of Ekottarika-āgama Studies — on Mahāyāna, Hīnayāna, and Theravāda respectively — are worth reading early on: they show, concretely, where the boundary between "early" and "later" Buddhist material actually gets drawn in a single collection, and how terms like mahāyāna and hīnayāna surface even within texts otherwise considered part of the early discourse literature. Read alongside the Madhyama-āgama appendix on school affiliation, they give a clear sense of how the later schools — including the Theravāda tradition itself — are best understood as downstream elaborations of a shared, older body of material, rather than as the origin of that material.

Anālayo himself gives explicit guidance on reading order in the introduction to Dīrgha-āgama Studies: although it was published last, he recommends it be read first, followed by Madhyama-āgama Studies, Saṃyukta-āgama Studies, and finally Ekottarika-āgama Studies — broadly the same sequence in which the four collections themselves are traditionally ordered.

These four introductions, read together, are effectively a short course on the four Āgamas: their translators, likely school affiliations, structure, and what each collection is thought to emphasize.

These three chapters take up the significance of the aṅgas (formal categories of early Buddhist text) and how discourses came to be sorted into āgamas or nikāyas in the first place — essential background for understanding why parallel versions differ as much as they do.

Once the framework is in place, the individual chapters — each built around a translation of one discourse and a close comparative discussion — become far more legible. Reading a Pāli discourse's summary alongside its Āgama counterpart, discourse by discourse, is where the method described above is actually put into practice.