先天功(gong),武俠小說中絕學。《射雕英雄傳(chuan)》中是「中神(shen)通(tong)」王重陽的絕學。而其他書中解釋為呼吸吐納的練功(gong)之法(fa),講(jiang)究抱元守(shou)一,欲練此(ci)功(gong),身如(ru)處子,不動如(ru)山,動如(ru)脫兔,呼先天下(xia)之憂而憂,后天下(xia)之樂(le)(le)而樂(le)(le)。修真(zhen)活(huo)計(ji)有何憑(ping),心(xin)死群情今(jin)不生。精氣充盈功(gong)行具,靈光照(zhao)耀滿神(shen)京,秘語師傳(chuan)悟本初,來時(shi)無(wu)欠去(qu)無(wu)余(yu)。神(shen)則守(shou),身則守(shou),此(ci)功(gong)乃道家獨門不傳(chuan)之秘,以(yi)祛(qu)百病、調虛(xu)實(shi),治(zhi)療(liao)沉重內(nei)傷。此(ci)功(gong)極耗內(nei)力,需以(yi)高深(shen)內(nei)功(gong)為輔方可(ke)學之,當達于(yu)極限(xian)時(shi),能相觸無(wu)聲,空中暗勁橫流,風力激蕩,功(gong)法(fa)里(li)的奧(ao)秘能使人潛能意識被激發到極限(xian),武學自會步入深(shen)不可(ke)測之化境。
在金庸最初(chu)的(de)(de)小(xiao)說(shuo)(shuo)創作(zuo)中(zhong),「先天(tian)功(gong)」是「南帝(di)」段(duan)智(zhi)興(xing)的(de)(de)絕(jue)(jue)學(xue),而「中(zhong)神(shen)通」王(wang)重陽(yang)(yang)的(de)(de)絕(jue)(jue)學(xue)卻(que)是「一(yi)(yi)陽(yang)(yang)指(zhi)」。后來創作(zuo)《天(tian)龍(long)(long)八(ba)部(bu)》將「一(yi)(yi)陽(yang)(yang)指(zhi)」改成(cheng)(cheng)「大理段(duan)氏」武(wu)學(xue)。因此,在修訂時(shi)(shi)《射雕英雄傳》、《神(shen)雕俠侶》、《倚天(tian)屠龍(long)(long)記》、《天(tian)龍(long)(long)八(ba)部(bu)》這(zhe)四部(bu)小(xiao)說(shuo)(shuo)時(shi)(shi),為了(le)能(neng)夠前後呼應,互相對照,特意(yi)將王(wang)重陽(yang)(yang)與(yu)段(duan)智(zhi)興(xing)的(de)(de)絕(jue)(jue)技交換,曾打敗「西(xi)毒」歐陽(yang)(yang)鋒(feng)(feng)的(de)(de)王(wang)重陽(yang)(yang)傳之以(yi)法,將「先天(tian)功(gong)」以(yi)「一(yi)(yi)陽(yang)(yang)指(zhi)」使出,以(yi)剋制「蛤蟆(ma)功(gong)」,以(yi)免(mian)造成(cheng)(cheng)段(duan)智(zhi)興(xing)在第(di)一(yi)(yi)次(ci)「華山論(lun)劍」時(shi)(shi),身負能(neng)夠克制「西(xi)毒」歐陽(yang)(yang)鋒(feng)(feng)的(de)(de)武(wu)功(gong)卻(que)只(zhi)是與(yu)「西(xi)毒」戰(zhan)成(cheng)(cheng)平手的(de)(de)古怪(guai)結果。王(wang)重陽(yang)(yang)徒弟「全真七子(zi)」跟師弟「老頑童(tong)」周(zhou)伯通均未(wei)獲得傳授(shou),僅(jin)與(yu)「南帝(di)」段(duan)智(zhi)興(xing)交換過「一(yi)(yi)陽(yang)(yang)指(zhi)」,用意(yi)是要「南帝(di)」擁有剋制「西(xi)毒」歐陽(yang)(yang)鋒(feng)(feng)之能(neng)。「先天(tian)功(gong)」配合「一(yi)(yi)陽(yang)(yang)指(zhi)」除(chu)了(le)可(ke)以(yi)完(wan)全對付歐陽(yang)(yang)鋒(feng)(feng)外,也可(ke)以(yi)祛百病、調虛實,治療(liao)沉(chen)重內傷。
在不(bu)(bu)同(tong)(tong)小說(shuo)描(miao)述里有(you)不(bu)(bu)同(tong)(tong)作(zuo)用(yong),上(shang)官鼎(ding)小說(shuo)《沉(chen)沙谷》描(miao)述:「全(quan)真(zhen)教(jiao)」和(he)「少(shao)(shao)林派(pai)」是(shi)最敝帚自珍,天(tian)下所矚目的(de)(de)(de)就是(shi)「先(xian)天(tian)罡氣」。全(quan)真(zhen)與少(shao)(shao)林雖(sui)然(ran)在取勁上(shang)有(you)柔剛之(zhi)(zhi)別,但天(tian)下的(de)(de)(de)武(wu)學是(shi)萬變不(bu)(bu)離其宗,而(er)以(yi)「人」為原則(ze)的(de)(de)(de)。況(kuang)且,兩派(pai)的(de)(de)(de)「先(xian)天(tian)氣功(gong)(gong)」又是(shi)同(tong)(tong)屬一個范圍(wei)的(de)(de)(de)事物(wu)。許多「全(quan)真(zhen)派(pai)」的(de)(de)(de)功(gong)(gong)訣上(shang)不(bu)(bu)夠份量的(de)(de)(de)地方(fang),拿「少(shao)(shao)林心(xin)法」一加對照,便瞭然(ran)於胸了,而(er)相反的(de)(de)(de),「少(shao)(shao)林心(xin)法」的(de)(de)(de)缺點也可以(yi)用(yong)全(quan)真(zhen)之(zhi)(zhi)長來(lai)補救。少(shao)(shao)林和(he)全(quan)真(zhen)佛道兩門至高武(wu)學水乳交融,少(shao)(shao)林和(he)全(quan)真(zhen)的(de)(de)(de)心(xin)法一旦合(he)并,舉(ju)世無(wu)敵(di)!使(shi)用(yong)者(zhe):陸(lu)介《長干行》描(miao)述:流(liu)落於關外「天(tian)池派(pai)」。當(dang)年「全(quan)真(zhen)教(jiao)」仗以(yi)威(wei)攝(she)武(wu)林,幾達(da)百馀年之(zhi)(zhi)久,後來(lai)全(quan)真(zhen)沒落,這種武(wu)功(gong)(gong)失傳(chuan),方(fang)有(you)少(shao)(shao)林武(wu)當(dang)等派(pai)興起,算(suan)起來(lai),這「先(xian)天(tian)功(gong)(gong)」正是(shi)武(wu)學之(zhi)(zhi)源。當(dang)掌力(li)達(da)於極限時(shi),能相觸(chu)無(wu)聲(sheng),卻最易傷(shang)人內腑,這種功(gong)(gong)夫,有(you)個名稱叫做「否(fou)極泰來(lai)」,正是(shi)物(wu)極必反的(de)(de)(de)道理,掌力(li)能不(bu)(bu)聞一些聲(sheng)響,空中暗勁橫流(liu),風(feng)力(li)激蕩。使(shi)用(yong)者(zhe):「定天(tian)一戟」高戰。