Genesis 3:22

And the Lord God said to the angels who ministered before Him, Behold, Adam is sole on the earth, as I am sole in the heavens above; and it will be that they will arise from him who will know to discern between good and evil. Had he kept the commandments which I appointed to him, he would have lived and subsisted as the tree of life for ever. But now, because he hath not kept that which I prescribed, it is decreed against him that we keep him from the garden of Eden, before he reach forth his hand and take of the tree of life: for, behold, if he eat thereof, living he will live and subsist for ever.

AND NOW, LEST HE PUT FORTH HIS HAND. The Holy One, blessed be He, wanted His decree concerning the death of Adam to be fulfilled, and if he were to eat of the tree of life which was created to give everlasting life to those who ate of its fruit, the decree would be nullified; for either he would not die at all or his day of death would not come at the time it was decreed for him and his descendants to die. And now that Adam had the power of choice, He therefore guarded this tree from him for at first Adam did only what he was commanded and he did not eat thereof as he did not need it.
Know and believe that the garden of Eden is on this earth that the garden….’” as are also the tree of life and the tree of knowledge, and from there the river comes forth and is divided into four heads which are visible to us. For the Euphrates, 2:14. is in our land and within our border, and Pishon, according to the words of the former scholars, is the Nile of Egypt. But as these are on earth so are there also in the heavens things similarly named, and those in the heavens are the foundations of these on earth, just as the Rabbis have said: “The king hath brought me into his chambers — this teaches us that the Holy One, blessed be He, is destined to show Israel the treasures on high that are chambered in the heavens. Another interpretation of The king hath brought me into his chambers is that these are the chambers of the garden of Eden. It is on the basis of this that they have said: ‘The work of the garden of Eden is like the work of the firmament.’” The rivers correspond to the four camps of angels on high, and it is from there that the power of the kingdoms on earth is derived, just as it is written, The host of the high heaven on high, and the kings of the earth upon the earth. Thus the Rabbis said in Bereshith Rabbah, “Into four heads — these are the four kingdoms. The name of the first is Pishon, 2:11. — this is Babylon, etc.” And the things called the tree of life and the tree of knowledge on high — their secret is high and lofty. Adam sinned with the fruit of the tree of knowledge below and on high, in deed and thought.
Now if the fruit of the tree were good for food and he desired it to become wise, why did He withhold it from him? Indeed, G-d is kind and dealeth kindly; He will withhold no good thing from them that walk uprightly! The serpent, moreover, has today no speaking faculty, and if it did have …

כאחד ממנו לדעת טוב ורע, he will know good and evil even while continuing to wear “our image.” This would be an intolerable situation, as in spite of his tendency to give in to his evil urge he would live on forever. [the author considers בצלמנו as a reference to the infinite life prevailing among the celestial beings. Ed.] In such circumstances, this Adam whose evil urge was active would continue to chase the material blessings of this world, something which would prevent him from reaching the spiritual aims set for him on earth when G’d made him in the divine image.

ויאמר ה' אלוקים...כאחד ממנו, G’d included Himself when speaking about the angels, just as He had done when about to create man in 1,26, when He had saidנעשה אדם בצלמנו, “Let Us make man in Our image.”

לדעת טוב ורע, we already explained that the term לדעת טוב ורע refers to perceptive powers equal to that of the angels in our commentary on 2,17. In Bereshit Rabbah 21,5 Rabbi Pappus explained the words כאחד ממנו to mean “like one of the ministering angels.” Rabbi Akiva violently disagreed with him, saying “you have overstepped the boundaries of permissible interpretation, i.e. דייך פפוס. What then is the meaning of these words? G’d has placed two paths in front of man, one is the path of life. If he chooses the other path, he automatically abandons the path of life.
Rabbi Berechyah, quoting Rabbi Yochanan, said that as long as man was single he was similar to the “One,” i.e. immortal. The moment his one side had been removed he became a split personality, i.e. that is the meaning of “knowing good and evil.” The opinion of the Onkelos sides with that of Rabbi Akiva who interpreted the word כאחד as being similar to the One and only One, having received his knowledge of good and evil directly from Him. In other words, his choice over good and evil stemmed directly from G’d, not from the fruit of the tree of knowledge.

כאחד, the word achad spelled with the vowel patach does not necessarily have to be in a construct form. Ibn Ezra does not agree that this is grammatically possible.