Well it's obvious to me that some "a priori" knowledge exists in the form of instinct. Instinctual knowledge can be readily observed with lower animals.
Man may demonstrate less instinctual knowledge as well as ability than many other animals at birth. But he is a more complex creature, requiring more time to develop.
As far as knowledge of right and wrong, I suppose that the biggest factor making me believe in an innate conscience as opposed to taught values is scripture. Romans 2 (at end of post) clearly states this is the case.
Which of course raises the question, "Why do I believe in scripture?". For the moment, I will simply say that it is a combination of personal experience, logic, the purity of the goal of human conduct that scriptures promotes, and it's ability to change human behavior. I will try to give a more detailed explanation for that belief later.
And it's actually going to get more complicated than that as I believe the conscience is not only internal innate knowledge but at times includes external subconscious communications with God and angels. Such belief also being based in part on scripture.
While I believe that man is born with a knowledge of right and wrong, he is also born with a fallen nature which makes him somewhat rebellious. Thus all men will eventually test the boundaries and do what they know to be wrong.
The rebellious nature is inherited, but the decision to do wrong invariably deals with man's desires. The Garden of Eden being a good example, Eve ate of the fruit of the tree of knowledge of good and evil because she saw that the fruit was 1) good for food, 2) a delight to the eyes, 3) and would make her wise (more like God by having knowledge of good and evil).
Eve also had a desire to obey God and to not die. The former was overridden by the focus that she had on the benefits of the fruit as well as the serpent planting distrust of God. The latter was overridden by accepting the Serpent's lie that Eve would not die and again the distrust of God.
Had Eve been a logic expert she would have realized that having already corrected the Serpent once, the serpent's information was more likely to be faulty than's God's. But she didn't, and here we are. The moral of the story being "who do you trust?" and "given conflicting information who do you obey?"
All acts of man, whether good or evil are driven thus by man's desires. The values and beliefs that we hold are a subset of those desires. The study of philosophy and values develops a construct where one can rank order or otherwise develop a greater knowledge of those desires and develop value rules or morals. That study and reflection of one's desires allow the desires themselves to change.
Romans2:14 When Gentiles who have not the law do by nature what the law requires, they are a law to themselves, even though they do not have the law. 15 They show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them 16 on that day when, according to my gospel, God judges the secrets of men by Christ Jesus. Which of course is going to raise the question of why I believe in scripture.
First let me say, most of what you had to say was what I would call, "theological," in nature, and needs to be addressed, from my point of view, separately from philosophical issues. One of the historical problems with philosophy for me has been the unfortunate mixture of the theological and philosophical. I know the reasons for it, but that it was a mistake should have been identified long ago.
Philosophy must deal with what can be known about man and nature based only on what one can be directly conscious of, so must include in it's realm only what is directly perceived, that it, material existense, all internal conscious experience, and all that can be deduced from these.
With regard to philosophy, there is no basis for assuming innate or a priori ideas. All knowledge can be deduced from the observable, including all values.
With that as a premise, let me address quickly some of your statements as to whether I agree or disagree:
Well it's obvious to me that some "a priori" knowledge exists in the form of instinct.
I do not agree that human beings have any instinct. Instinct is a pre-programmed pattern of behavior that determines all a creatures behavior, in those that have it. It determines what a creature must do to survive and automatically causes the animal to do those things, like seeking the right food, right environment, and right shelter.
Human bings have no automatic patterns of behavior (a reflex is not an instinct). Everything a human being does must be done by conscious choice. Nothing that a human being must do to survive or enjoy his life is given. It all must be discovered and learned, from what he must eat and how to acquire it to how to make medicine that cures his diseases. The rational/volitional nature excludes the possibility of instinct.
Now we come to a theological part:
While I believe that man is born ... with a fallen nature...
You said that you believe the Bible, but the "fallen nature," "original sin," doctrine was invented by Augustine, (and has been pretty much swallowed by most theologians since. It is not a Scriptural teaching, even though several Scriptures are invoked in support of it.
...the decision to do wrong invariably deals with man's desires. That is correct, both philosophically and Scripturally. Jas 1:14-15 "But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death" Here I quote an older post:
First, something about the word "lust." The word means "desire" and nothing more. It is the very same word used in Luke 22:15, "And he said unto them, With desire I have desired to eat this passover with you before I suffer, and could have rightly been translated, "... with lust I have lusted to eat this passover with you...."
With that understanding, we can see James is describing how all men are tempted. It begins with desire, not sinful or evil desire, but perfectly natural God-given desiress like the desire for food, or beauty, or comfort. Now these are the source of temptation, but not always, and even when they are, they are not sin.
The God-given natural desires for food, for beauty, and for knowledge Adam and Eve freely indulged and enjoyed in all the blessings of paradise, nevertheless those same desires became the source of temptation when their object was the forbidden fruit of the tree of knowledge of good and evil. Was there anything evil about the desires? Of course not. Then how could indulging them be sin? Because indulging them required disobedience. The temptation consisted entirely of this, there was a desired object, there was the knowledge that the object was forbidden (and therefore it would be wrong to fulfill that desire), and they had the ability to choose. It was temptation because to not sin they had to choose what they knew was right against what they desired and wanted.
Back to you:
The rebellious nature is inherited...
If it is, Jesus inherited the same nature. You do not believe that, but examine the Scriptures. Jesus had exactly the same nature we have, or he was not a man.
Again I quote my old post:
Heb 2:10-18 For it became him, for whom are all things, and by whom are all things, in bringing many sons unto glory, to make the captain of their salvation perfect through sufferings. For both he that sanctifieth and they who are sanctified are all of one: for which cause he is not ashamed to call them brethren, Saying, I will declare thy name unto my brethren, in the midst of the church will I sing praise unto thee. And again, I will put my trust in him. And again, Behold I and the children which God hath given me. Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil; And deliver them who through fear of death were all their lifetime subject to bondage. For verily he took not on him the nature of angels; but he took on him the seed of Abraham. Wherefore in all things it behoved him to be made like unto his brethren, that he might be a merciful and faithful high priest in things pertaining to God, to make reconciliation for the sins of the people. For in that he himself hath suffered being tempted, he is able to succour them that are tempted.
Consider:
"For both he that sanctifieth and they who are sanctified are all of one..." All one what? Why, one nature, of course, as is explained.
"Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same." The same what? Why, the same flesh and blood with the same nature, because if it had a different nature, it would not be the same flesh and blood.
"he took not on him the nature of angels; but he took on him the seed of Abraham. See, he's talking about nature here and plainly states that nature is the nature inherited from the SEED OF ABRAHAM. (Oh, yeah, almost forgot, you believe that seed is corrupt, I mean, sinfully. Can't be, else Jesus would not have inherited it.
"in all things it behoved him to be made like unto his brethren." That's in ALL things, including their nature. Of course if it did not include their nature, He would only have made like his brethren in "some" things.
Rom. 8:3 For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh
Phil. 2:7 But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men.
Here are two very interesting verses. They say Jesus was made "in the likeness of sinful flesh," and "in the likeness of men." Now you might want to get out of admitting the Bible teaches Jesus had the same kind of "sinful flesh" all other men have by claiming it says Jesus flesh was only "like" sinful flesh, but not really sinful flesh because is says "in the likeness of." If you do that, however, you are also going to have to say Jesus was only "like" a man, but not really a man because it says, "in the likeness of men." But of course you won't do that, because you know Jesus was a man and had exactly the same kind of nature all men have.
Heb 4:15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
Finally we must examine this verse:
Rom. 5:12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned.
I usually refrain from saying this, because it is so painfully obvious, I am embarrased to have to point it out. But, I think it is needed here.
This passage is frequently used to show that man has a sinful nature based on the idea that death is the result of sin, and since death is the result of Adam's sin, and death has passed on to all men (we are mortal), than sin must have passed on to all men as well, in what is called the "sinful nature."
On the basis of this view, every human death is proof of the sinful nature that man was born with. (I have actually seen this statement made.) Now, the obvious and absolute refutation of this is the fact that JESUS DIED.
To die, Jesus, had to have the same kind of nature we have, that is, not sinful, but mortal.
I think that should give you something to chew on.
(This is only one aspect of the "sunful nature" heresy that is easy to refute from Scripture. All men have sinned, not because of something the inherited, but by choice, just like Adam and Eve.)
Hank