The rendering of the Pali term yoniso poses a number of difficulties, some of which which I have discussed in previous posts. I will mention again the major issues, together with a new text that well illustrates the main point.
It is from the root yoni, meaning “womb” and by extension “source”. It seems that the non-Buddhist sources only know this meaning, and that the usage in Buddhism is quite distinct.
In the common form, it takes an instrumental ending, conveying the sense “by way of cause, cause-wise”. However, it is applied very broadly in the Buddhist texts, and moved towards having a more vague sense of “careful, wise, appropriate”, which is how it usually translated.
While this is not incorrect, it has long seemed to me that this lost some of the clarity of focus found in the early usages. Reviewing its occurrence over many passages, it seems to me that it is still strongly associated with passages dealing with causality, and that the sense of “in accordance with cause and effect” is quite prominent. Of course, this is, from a Buddhist point of view, ‘wise, careful, appropriate”, but it is more specific than that.
I just translated MN 126, which gives a series of similes that show the causal use of yoniso. Several everyday activities are described, each of which is given first in a negative form, showing the silly means that someone might use, which obviously will not produce the result they seek. For example, a person seeking oil tries to press it from sand; someone wanting milk pulls on a cow’s horn; or someone wanting butter churns water. Given that these absurd scenarios are mean as similes for the foolishness of wrong-headed spiritual practitioners, I can’t help thinking they would have raised a chuckle among the monks.
Each of these is described as ayoniso, just as someone practicing with wrong view, etc., will never see the fruits of the path.
Next the text gives the right method of doing each of these, showing how to extract oil from sesame, churn milk to get butter, and so on. These are all yoniso, just like a practitioner who applies right view, etc.
It seems clear to me that the sense of yoniso here is the causes are in line with the effects. I.e., the acts you perform actually produce the results you’re looking for. So the suitable English translations would be “rational” or “reasonable”.