先天(tian)功(gong)(gong),武俠(xia)小說中(zhong)絕學(xue)。《射雕(diao)英雄傳(chuan)(chuan)》中(zhong)是(shi)「中(zhong)神(shen)通」王重陽的絕學(xue)。而(er)其(qi)他書(shu)中(zhong)解釋為(wei)呼(hu)吸吐(tu)納的練功(gong)(gong)之(zhi)法,講究抱元守(shou)一,欲練此(ci)功(gong)(gong),身(shen)如(ru)處子(zi),不(bu)動如(ru)山,動如(ru)脫(tuo)兔,呼(hu)先天(tian)下之(zhi)憂而(er)憂,后天(tian)下之(zhi)樂(le)而(er)樂(le)。修真(zhen)活(huo)計有何憑,心死群情今不(bu)生(sheng)。精(jing)氣(qi)充盈功(gong)(gong)行具,靈光照耀(yao)滿神(shen)京(jing),秘語師傳(chuan)(chuan)悟本初(chu),來時無(wu)(wu)欠去無(wu)(wu)余(yu)。神(shen)則守(shou),身(shen)則守(shou),此(ci)功(gong)(gong)乃道家(jia)獨門不(bu)傳(chuan)(chuan)之(zhi)秘,以祛百病、調(diao)虛實,治療沉重內傷。此(ci)功(gong)(gong)極(ji)(ji)(ji)耗內力,需以高深(shen)內功(gong)(gong)為(wei)輔(fu)方可(ke)學(xue)之(zhi),當達于極(ji)(ji)(ji)限時,能相觸(chu)無(wu)(wu)聲,空中(zhong)暗勁橫流,風力激(ji)蕩,功(gong)(gong)法里的奧(ao)秘能使(shi)人潛能意(yi)識被激(ji)發到(dao)極(ji)(ji)(ji)限,武學(xue)自會步入深(shen)不(bu)可(ke)測之(zhi)化(hua)境。
在(zai)(zai)金庸最初的(de)小(xiao)說創(chuang)作(zuo)中,「先(xian)天功」是(shi)「南(nan)帝」段(duan)(duan)智(zhi)興的(de)絕學,而「中神通」王(wang)重(zhong)陽(yang)(yang)的(de)絕學卻是(shi)「一(yi)陽(yang)(yang)指」。后來創(chuang)作(zuo)《天龍八(ba)部(bu)》將(jiang)(jiang)「一(yi)陽(yang)(yang)指」改成(cheng)(cheng)「大理段(duan)(duan)氏」武學。因此(ci),在(zai)(zai)修訂時(shi)(shi)《射雕英雄傳(chuan)》、《神雕俠侶》、《倚(yi)天屠龍記》、《天龍八(ba)部(bu)》這(zhe)四部(bu)小(xiao)說時(shi)(shi),為了(le)能夠(gou)前(qian)後(hou)呼(hu)應(ying),互相對照,特意(yi)將(jiang)(jiang)王(wang)重(zhong)陽(yang)(yang)與段(duan)(duan)智(zhi)興的(de)絕技交換(huan),曾打敗(bai)「西毒(du)(du)」歐(ou)陽(yang)(yang)鋒的(de)王(wang)重(zhong)陽(yang)(yang)傳(chuan)之(zhi)以(yi)(yi)法,將(jiang)(jiang)「先(xian)天功」以(yi)(yi)「一(yi)陽(yang)(yang)指」使出,以(yi)(yi)剋(ke)制(zhi)(zhi)「蛤(ha)蟆功」,以(yi)(yi)免(mian)造成(cheng)(cheng)段(duan)(duan)智(zhi)興在(zai)(zai)第一(yi)次「華山論(lun)劍」時(shi)(shi),身負(fu)能夠(gou)克制(zhi)(zhi)「西毒(du)(du)」歐(ou)陽(yang)(yang)鋒的(de)武功卻只是(shi)與「西毒(du)(du)」戰成(cheng)(cheng)平手的(de)古怪(guai)結果。王(wang)重(zhong)陽(yang)(yang)徒弟(di)「全真(zhen)七子」跟師(shi)弟(di)「老(lao)頑童」周伯(bo)通均未(wei)獲得傳(chuan)授,僅與「南(nan)帝」段(duan)(duan)智(zhi)興交換(huan)過「一(yi)陽(yang)(yang)指」,用(yong)意(yi)是(shi)要「南(nan)帝」擁有剋(ke)制(zhi)(zhi)「西毒(du)(du)」歐(ou)陽(yang)(yang)鋒之(zhi)能。「先(xian)天功」配合「一(yi)陽(yang)(yang)指」除了(le)可以(yi)(yi)完全對付歐(ou)陽(yang)(yang)鋒外,也可以(yi)(yi)祛(qu)百病、調(diao)虛實,治療沉重(zhong)內傷(shang)。
在(zai)不(bu)同小說描述(shu)里有(you)(you)不(bu)同作用(yong),上(shang)官鼎(ding)小說《沉沙谷》描述(shu):「全(quan)(quan)(quan)真(zhen)(zhen)(zhen)教」和(he)「少(shao)林(lin)(lin)派(pai)」是(shi)最敝帚自珍,天(tian)下所(suo)矚目(mu)的(de)(de)(de)就(jiu)是(shi)「先天(tian)罡氣(qi)」。全(quan)(quan)(quan)真(zhen)(zhen)(zhen)與(yu)少(shao)林(lin)(lin)雖(sui)然(ran)在(zai)取(qu)勁(jing)上(shang)有(you)(you)柔剛之別,但天(tian)下的(de)(de)(de)武學(xue)是(shi)萬變不(bu)離其宗,而以(yi)「人」為原(yuan)則的(de)(de)(de)。況且,兩派(pai)的(de)(de)(de)「先天(tian)氣(qi)功(gong)(gong)」又(you)是(shi)同屬一(yi)(yi)個范圍的(de)(de)(de)事物。許多「全(quan)(quan)(quan)真(zhen)(zhen)(zhen)派(pai)」的(de)(de)(de)功(gong)(gong)訣上(shang)不(bu)夠份量的(de)(de)(de)地方,拿「少(shao)林(lin)(lin)心(xin)法(fa)」一(yi)(yi)加對照,便瞭然(ran)於胸(xiong)了(le),而相(xiang)反的(de)(de)(de),「少(shao)林(lin)(lin)心(xin)法(fa)」的(de)(de)(de)缺點也(ye)可以(yi)用(yong)全(quan)(quan)(quan)真(zhen)(zhen)(zhen)之長來補(bu)救。少(shao)林(lin)(lin)和(he)全(quan)(quan)(quan)真(zhen)(zhen)(zhen)佛(fo)道兩門至高(gao)武學(xue)水乳(ru)交(jiao)融,少(shao)林(lin)(lin)和(he)全(quan)(quan)(quan)真(zhen)(zhen)(zhen)的(de)(de)(de)心(xin)法(fa)一(yi)(yi)旦合(he)并,舉(ju)世無敵!使用(yong)者(zhe):陸(lu)介《長干(gan)行》描述(shu):流落(luo)(luo)於關(guan)外「天(tian)池派(pai)」。當(dang)年「全(quan)(quan)(quan)真(zhen)(zhen)(zhen)教」仗以(yi)威攝武林(lin)(lin),幾達百(bai)馀年之久,後來全(quan)(quan)(quan)真(zhen)(zhen)(zhen)沒落(luo)(luo),這種武功(gong)(gong)失傳,方有(you)(you)少(shao)林(lin)(lin)武當(dang)等(deng)派(pai)興起(qi),算(suan)起(qi)來,這「先天(tian)功(gong)(gong)」正是(shi)武學(xue)之源。當(dang)掌力達於極限時,能(neng)相(xiang)觸無聲(sheng),卻最易傷人內(nei)腑,這種功(gong)(gong)夫,有(you)(you)個名稱(cheng)叫(jiao)做「否(fou)極泰來」,正是(shi)物極必反的(de)(de)(de)道理,掌力能(neng)不(bu)聞一(yi)(yi)些聲(sheng)響,空中暗勁(jing)橫流,風力激蕩(dang)。使用(yong)者(zhe):「定天(tian)一(yi)(yi)戟」高(gao)戰。