The root gno- ((g)nosco, know, kennen) conveys the idea, not so much of knowledge in itself, as of the act of perception by which knowledge is acquired.  It means to perceive or become aware of a fact, to distinguish an object, to recognise a person, as being what they are, from their proper marks or characteristics.  Thus, to give but a single example from the classics, when Aeacus is unable to distinguish between the god Dionysus and his attendant Xanthias, he conducts them to his master, Pluto.  o` despo,thj ga.r auvtoj u`ma/j gnw,setai:  "For the master himself will know you, i.e. "will discern what you really are " (Ar. Ran. 670).

    In the different tenses of the verb, this root-idea assumes corresponding shades of meaning.  The reduplicate form of the present gi(g)inw,skein signifies durative action, - to have continuous perception of the object, to be acquiring knowledge of it; the aorist gnw/nai, the act of perception and its immediate result, - to become aware of, ascertain, realise;  the perfect evgnwke,nai, the act with its result down to the present time, - to have learned, to have become acquainted with, and, therefore, to know.  The knowledge acquired has become a permanent possession.

    A few illustrative examples may be taken from the Fourth Gospel:

(a)  Present and imperfect. - po,qen me givw,skeij (1⁴⁹) = By what means do you know me, i.e. read my character (as an Israelite indeed)?  dia. to. auvto.n ginw,skein pa,ntaj . . . auvto.j ga.r evgi,nwske ti, h=n evn tw/| avnqrw,pw| (2^24,25) = By reason of His discerning the real character of all men . . . for He always perceived what was in man.  ginw,skw ta. evma,, kai. ginw,skousi, me ta. evma, (10¹⁴).  The Good Shepherd recognises His own sheep, and they recognise Him.

(b)  Aorist. - w`j ou=n e;gnw o` ku,rioj o[ti (4¹; cf. 4⁵³) = When, therefore, the Lord became aware that.  gnou.j o[ti polu.n h;dh cro,non (5⁶) = Noticing (from the man's appearance) that he had been a long time.

(c) Perfect. - kai. ouvk e;gnwka,j me; (14⁹) = Hast thou not recognised (and so, dost thou not yet know) who and what I am?

Present.  1.  ginw,skein signifies the perception or recognition of a person.  o` ko,smoj ouv ginw,skei h`ma/j (3¹).  (We are the children of God, but) the world does not recognise us as being what we are.  o` ginw,skwn to.n qeo,n (4⁶) = He that recognises the Divine when it is presented to him.  ginw,skei to.n qeo,n (4⁷) = (Only he that loves) has a true perception of the character of God. ginw,skwmen to.n avlhqino,n (5²⁰).  (By the understanding given us) we recognise the True One (in contradistinction to "idols," 5²¹).

2. The perception or recognition of a thing.  evn tou,tw| ginw,skete to. pneu/ma tou/ qeou/ (4²) = By this recognise the Spirit of God (in distinction from other spirits).  evk tou,tou ginw,skomen to. pneu/ma
th/j avlhqei,aj kai. to. pneu/ma th/j pla,nhj (4⁶) = By this token we recognise the Spirit of Truth and the spirit of error.  o` qeo,j . . . ginw,skei pa,nta (3²⁰) = God observes all things is aware of them and discerns their true character.

3.  The inferential perception of a fact from the proofs of its existence.  evn tou,tw| ginw,skomen o[ti (2^3,5, 3^19,24, 4¹³, 5²) = By this we recognise that the fact is so and so.  Similarly o[qen ginw,skomen o[ti (2¹⁸).  ginw,skete o[ti (2²⁹) = (If ye know, as ye do, that God is righteous) recognise the consequence that every one also that doeth righteousness, etc.

Aorist. - o[ti ouvk e;gnw auvto,n (3¹) = (The world does not recognise us because) it did not recognise Him (pointing to the definite time when it failed to do so, namely, when He was manifested on earth.  Or, the force of the aorist here may be the same as in the following example).  o` mh. avgapw/n ouvk e;gnw to.n qeo,n (4⁸).  Here the aorist gathers to one point the whole extent of the failure to perceive what God is (cf. John 17²⁵), and ouvk e;gnw may be translated "has never known."  (This perfective sense of the aorist is shared by the past tense in English.  "I never knew such a rascal" = I have never known until now such a rascal.)

Perfect. - 1.  Is used of persons, signifying perception of and acquaintance with their character.  evgnw,kamen auvto,n (2^3,4).  o[ti evgnw,kate to.n avpV avrch/j . . . to.n pate,ra (2^13,14).  The tense connotes that the spiritual perception of the object, which is always God or Christ, has become a permanent experience.  An instructive case is o` a`marta,nwn ouvc e`w,raken auvto.n ouvde. e;gnwken auvto,n (3⁶) = He that sinneth hath not seen Christ, nor had any true perception of Him at all.

2.  It is used of things in the same sense as of persons.  evn tou,tw| evgnw,kamen th.n avga,phn, o[ti . . . (3¹⁶).  We have learned to know what love is by this that . . .  kai. h`mei/j evgnw,kamen kai. pepisteu,kamen th.n avga,phn (4¹⁶) = We have perceived (come to know) the Love, and are persuaded of its reality.

    While ginw,skein always suggests, more or less distinctly, the perception through which knowledge is acquired, eivde,nai, on the other hand, expresses the fact of knowledge absolutely.  It frequently happens, however, that the same experience may be stated from either point of view; and thus it is not possible, in actual usage, to draw any rigid line of distinction between the two.

    It may be noted that eivde,nai expresses -

1.  Knowledge of a fact, apart from consideration of how it is known.  oi;date pa,ntej (2²⁰), oi;date th.n avlh,qeian (2²¹).

2. Knowledge of self-evident or necessary truth.  eva.n eivdh/te o[ti di,kaio,j evstin ginw,skete . . . 2²⁹.   That God is righteous is self-evident - a matter of intuitive knowledge; that every one that doeth righteousness is begotten of Him is recognised only as a necessary consequence from this.  The same self-evident certainty is expressed by oi;damen in 3².  ("We know," beyond question, "that if He shall be manifested, we shall be like Him"), in 3⁵ ("Ye know," it is axiomatic, "that He was manifested to take away sins"), in 3¹⁵ ("Ye know that no murderer hath eternal life abiding in him"), in 5¹⁵ ("We know that He heareth" . . . "We know that we have ").  Cf. Rom. 6¹⁶, 8²⁸, I Cor. 3¹⁶, 6^9,15,19, Eph. 6^8,9, Col. 4¹, I Pet. 1¹⁸.

3.  It is equivalent to ginw,skein heightened by exultant emotion (3¹⁴, 5^13,18-20).

4.  It seems to be simply equivalent to ginw,skein (2¹¹).