In Theravada, is attaining mundane jhana considered easier than achieving stream-entry, or is it the other way around?
The way I understand the suttas from my limited perspective, stream entry, or sotapatti, is the first level out of four of THE goal: enlightenment.
Jhana, on the other hand, is a state of meditation that the Buddha created spontaneously while he was sitting during a harvest festival years before he left home and started the path of striving for enlightenment.
As to rarity, again, from my limited understanding of the suttas, jhana is a lesser attainment and is more common.
This is not to downplay the importance of jhana at all, however. It is only to say it may be more common than stream entry.
Mundane jhana is very difficult to attain, and far more difficult to master.
However stream entry is immeasurable more difficult.
Samyutta Nikaya (V, Mahavagga, Book XII, the Truths, chapter V), The keyhole.
"Now what think you, Ananda? Which which is the harder task : To shoot like that or to pierce one strand of hair, a hundred times divided, with another strand?
Why, lord, of course to split a hair in such a way is the harder, much the harder task.
Just so, Ananda, they who penetrate the meaning of: This is dukkha, this is the arising of dukkha, this is the ceasing of dukkha, this is the practice that leads to the ceasing of dukkha, pierce through something much harder to pierce.
and
Ariyapariyesana Sutta
"This Dhamma that I have attained is profound, hard to see and hard to understand, peaceful and sublime, unattainable by mere reasoning,
subtle, to be experienced by the wise.
I don’t know of the two being compared anywhere but logically of course jhana is easier to achieve, stream entry is basically more or less the end of the path and is extremely difficult to actually see, it literally takes a Buddha to show the way to it.
Jhana on the other hand is “just” concentration, albeit very difficult to achieve but still, jhana is within the domain of anyone, not just Buddhists, whereas stream entry is revealed only within a Buddha’s teaching.
Every powerful seer has had great concentration and mind control, but not insight unless he is a Buddha, Paccekabuddha or a Buddha’s disciple. Even ancient Egyptians who lived long before the Master knew for example that the mind was contained within the heart, which is knowable by just concentration.
In any case great concentration, whether access (which is very close to jhana) or jhana itself is necessary for insight, so it being a prerequisite (at least access concentration) logically makes it easier.
I agree with your post Vojislav. This part needs some clarification though. The concentration that is associated with insight comes with that special understanding that - as you noted- depends on hearing the teaching of the Buddhas. And at moments of satipatthana the level of concentration is not necessarily especially strong - although during the flashes of vipassana level insight it will be for those moments.