We must remember that we are dealing not with Buddha's later meditation under the Bodhi tree but his first effective Jhana meditation that revealed a potential future way of meditation.
Jhana meditation is customarily considered effective at three levels:
1. The inferior, since it is recently attained (Hina).
2. That of medium intensity (Majjhima), since it is not fully developed.
3. That which is superior because it is fully developed (Panita).
That first meditation of Buddha of the four Jhanas without the Brahmic Realization was the inferior variety. However, it was correctly developed for he was well-practiced in the traditional methods.
Let us speak of that correct development.
Paravahera Vajranana Mahathera gives us a clear exposition of the meditation to the fourth Jhana, when it is fully developed:
"He, with mind thus concentrated, purified, lucent, unstained, with the defilements gone, supple, ready to act, firm and imperturbable, turns and applies the meditation to the acquisition of knowledge, that which consists of higher knowledge and insight (Abhinna)."
Using the system of eight Jhana levels, not the nine levels of later Abhidhamma description, we know that the first Jhana of Buddha's Roseapple tree meditation at the first level contained Vitakka, Vicara, Zest and Joy. We can ask then what was his selected and organized topic of meditation?
We may intuit that as it was a glorious festival the Prince sat contemplating something simple, perhaps agreeable, like the Nature of Earth or the Sky. But his nature as a person of intelligence and aversion suggests that it was a more serious topic that perhaps troubled him.
Personally, I imagine it to be related to the three experiences of illness, old age and death which he had recently come across, when he exclaimed "And this, could it happen to me?," along with his own dread at his clinging reaction to phenomena at one level and his intellectual rejection at the other.
I imagine then that it was the topic of HUMAN SUFFERING, not yet divorced from his own recent and unexplainable experiences. He was not interested in the Brahmin realization nor in some divine knowledge. His interest was in Vijjha and Carana, that is, knowledge and conduct that was contrasted to his educational knowledge concerning arbitrary social rules and ceremonial purity.
And so, dwelling with this theme in Vitakka of the first Jhana, he left behind then all verbal conceptualizations, sensations, discriminations, perceptions; and dweling then in the sustaining support of Vicara, he then also allowed that also to fall away with the entry into the second Jhana.
Sa-Vitakka - Sa-Vicara-matta was then was deliberative and conceptual
A-Vitakka - Sa-Vicara-matta was non-deliberative, but conceptual
A-Vitakka - A-Vicara was non-deliberative and non-conceptual
Thus with Zest and Joy he continued his meditation, releasing each in turn until he reached the fourth Jhana, which allowed his mind to turn to a higher non-conceptual knowledge and insight (Abhinna) regarding Suffering that was beyond the last conceptualization and was within the unconscious language of functional experiences.
It was perhaps this first attempt, inferior though it was, that reinforced his desire to leave behind the Samsara he dreaded, leaving home, wife and child, seeking a greater truth than the Brahmic realizations could provide.