In his fictional biography, Caeiro was born in Lisbon on 16 April 1889, lived most his life in a village in Ribatejo and died in 1915.

He was the leader and teacher of a group of neopagan poets and intellectuals that included Pessoa's other heteronyms António Mora, Ricardo Reis and Álvaro de Campos.

The first and most famous work Pessoa composed under this name was The Keeper of Sheep [pt], a series of 49 poems he began in 1914 and continued to edit until his death in 1935.

Like Pessoa's works in general, the Caeiro poems began to receive high critical acclaim decades after the writer's death.

[1] Alberto Caeiro was the first of his major heteronyms, a term he used for what was a mixture of pen names, author personas and fictional characters.

Based partially on Pessoa's assertive denial, the scholar Richard Zenith argues that the influence from Whitman was considerable.

[11] He developed his own poetics based on free verse and a worldview that premiered the immediate experience and rejected attempts to find an underlying truth or meaning in things.

'General Programme of Portuguese Neo-Paganism'), through which Pessoa's heteronyms sought to explore neopaganism with Caeiro as their starting point.

[5] The Keeper of Sheep [pt] (Portuguese: O Guardador de Rebanhos) is a collection of 49 poems and the most famous of Caeiro's works.

The title is attributed to Caeiro, but some of the poems were first included in the collection by the literary critic Maria Aliete Galhoz.

[15] Pessoa prepared material for promoting Caeiro's works in Europe, writing his own reviews and an English-language introduction under the name I. I.

A bilingual English-Portuguese version of this book was published by New Directions in 2020 as The Complete Works of Alberto Caeiro, with English interpretations by Margaret Jull Costa and Ferrari.