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bodies are heavy, and therefore fall when their supports are withdrawn.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bodies are heavy, and therefore fall when their supports are withdrawn.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

Methinks thou dost pose a question of great intricacy, one that doth require deep contemplation and profound reflection. The lines speak of the distinction betwixt that which is known a priori, through pure reason, and that which is known a posteriori, through experience. It is true that the expression a priori doth not fully convey the essence of the matter, for oft have we claimed knowledge to be a priori when in truth it is but derived from empirical sources, albeit indirectly. Verily, we speak of knowing a truth a priori when we perceive it not through direct experience, but through a universal rule, a rule that itself is rooted in experience. Thus, we may say that a man knew a priori that his house would fall if its foundations were undermined, but this knowledge itself is derived from the empirical fact that bodies are heavy and fall when their supports are taken away. Therefore, true a priori knowledge is that which is entirely independent of all experience, free from any empirical influence. On the contrary, empirical knowledge is that which is gained only through experience, a posteriori. A priori knowledge may be deemed pure when it is devoid of any empirical elements, but even seemingly a priori truths may have their origins in experience. This distinction, my friends, is of utmost importance, for it shapes the very foundations of our understanding of the world around us. But let us not be hasty in our judgments, let us delve deeper into the realms of knowledge and seek to unravel the mysteries that lie before us with wisdom and discernment."