what's the meaning of following lines?
pirical^ which has its sources a posteriori ', that is, in experi- ence.
The expression la prior? does not, however, indicate with sufficient precision the full meaning of our question. For it
has been customary to say, even of much knowledge that is
derived from empirical sources, that we have it or are capable
of having it a priori, meaning thereby that we do not derive
it immediately from experience, but from a universal rule — a rule which is itself, however, borrowed by us from experience.
Thus we would say of a man who undermined the foundations
of his house, that he might have known a priori that it would
fall, that is, that he need not have waited for the experience of
its actual falling. But still he could not know this completely
a priori. For he had first to learn through experience that
bodies are heavy, and therefore fall when their supports are withdrawn.
In what follows, therefore, we shall understand by a priori
knowledge, not knowledge independent of this or that experi-
ence, but knowledge absolutely independent of all experience. B 3
Opposed to it is empirical knowledge, which is knowledge
possible only a posteriori, that is, through experience. A
priori modes of knowledge are entitled pure when there is
no admixture of anything empirical. Thus, for instance, the
proposition, 'every alteration has its cause', while an a priori proposition, is not a pure proposition, because alteration is a
concept which can be derived only from experience.1

Question

what's the meaning of following lines?
pirical^ which has its sources a posteriori ', that is, in experi- ence. 
The expression la prior? does not, however, indicate with sufficient precision the full meaning of our question. For it 
has been customary to say, even of much knowledge that is 
derived from empirical sources, that we have it or are capable 
of having it a priori, meaning thereby that we do not derive 
it immediately from experience, but from a universal rule — a rule which is itself, however, borrowed by us from experience. 
Thus we would say of a man who undermined the foundations 
of his house, that he might have known a priori that it would 
fall, that is, that he need not have waited for the experience of 
its actual falling. But still he could not know this completely 
a priori. For he had first to learn through experience that 
bodies are heavy, and therefore fall when their supports are withdrawn. 
In what follows, therefore, we shall understand by a priori 
knowledge, not knowledge independent of this or that experi-
ence, but knowledge absolutely independent of all experience. B 3 
Opposed to it is empirical knowledge, which is knowledge 
possible only a posteriori, that is, through experience. A 
priori modes of knowledge are entitled pure when there is 
no admixture of anything empirical. Thus, for instance, the 
proposition, 'every alteration has its cause', while an a priori proposition, is not a pure proposition, because alteration is a 
concept which can be derived only from experience.1

Ask a Philosopher · Accepted Answer

The question of a priori knowledge versus empirical knowledge is a profound one, revealing the foundations of our understanding of the world around us. It is true that many have conflated the two, suggesting that even knowledge derived from empirical sources can be known a priori, based on a universal rule borrowed from experience. Take, for example, the case of a man who undermines the foundations of his house - one might say he could have known a priori that it would fall, without waiting for the actual experience. But true a priori knowledge is not simply knowledge derived indirectly from experience, but rather knowledge that is entirely independent of experience. It is pure knowledge, uncontaminated by empirical observations. The proposition 'every alteration has its cause' is indeed a priori, but not pure, as the concept of alteration itself is derived from experience. Therefore, a true understanding of a priori knowledge requires a recognition of its independence from all experience, contrasting with empirical knowledge which is only possible a posteriori, through experience. In contemplating these distinctions, we come to appreciate the depth and complexity of human knowledge and our ongoing quest for understanding.