先(xian)天功(gong)(gong),武(wu)俠小(xiao)說中絕(jue)學。《射雕英雄傳》中是「中神(shen)通」王重陽的(de)(de)絕(jue)學。而(er)(er)其他書中解釋(shi)為呼(hu)吸吐納的(de)(de)練功(gong)(gong)之(zhi)(zhi)(zhi)法,講究抱元守一,欲練此功(gong)(gong),身(shen)如(ru)(ru)處子,不(bu)動(dong)如(ru)(ru)山,動(dong)如(ru)(ru)脫兔,呼(hu)先(xian)天下之(zhi)(zhi)(zhi)憂而(er)(er)憂,后天下之(zhi)(zhi)(zhi)樂而(er)(er)樂。修真活計(ji)有何憑,心(xin)死(si)群情今不(bu)生。精氣充盈功(gong)(gong)行具,靈光(guang)照耀滿神(shen)京,秘(mi)語師傳悟本初,來(lai)時無(wu)欠去無(wu)余(yu)。神(shen)則守,身(shen)則守,此功(gong)(gong)乃道家獨門(men)不(bu)傳之(zhi)(zhi)(zhi)秘(mi),以(yi)祛百病、調虛實(shi),治療沉(chen)重內(nei)傷。此功(gong)(gong)極耗內(nei)力,需以(yi)高(gao)深內(nei)功(gong)(gong)為輔方(fang)可學之(zhi)(zhi)(zhi),當達于極限時,能(neng)相觸無(wu)聲(sheng),空(kong)中暗勁橫流(liu),風力激蕩,功(gong)(gong)法里的(de)(de)奧秘(mi)能(neng)使人潛能(neng)意(yi)識(shi)被激發到極限,武(wu)學自會步(bu)入深不(bu)可測(ce)之(zhi)(zhi)(zhi)化境(jing)。
在金庸最初的(de)小說創作中,「先天(tian)(tian)功(gong)」是「南(nan)帝(di)」段(duan)(duan)智興(xing)的(de)絕(jue)學,而「中神通」王重(zhong)(zhong)陽(yang)(yang)(yang)的(de)絕(jue)學卻是「一(yi)陽(yang)(yang)(yang)指(zhi)(zhi)」。后來創作《天(tian)(tian)龍(long)八部(bu)》將(jiang)(jiang)「一(yi)陽(yang)(yang)(yang)指(zhi)(zhi)」改成「大理段(duan)(duan)氏(shi)」武(wu)學。因此(ci),在修訂時《射雕英(ying)雄(xiong)傳(chuan)》、《神雕俠侶》、《倚天(tian)(tian)屠龍(long)記》、《天(tian)(tian)龍(long)八部(bu)》這四部(bu)小說時,為了(le)能(neng)夠(gou)前(qian)後呼(hu)應,互(hu)相對照,特意(yi)將(jiang)(jiang)王重(zhong)(zhong)陽(yang)(yang)(yang)與(yu)段(duan)(duan)智興(xing)的(de)絕(jue)技交換(huan)(huan),曾打敗「西(xi)毒」歐陽(yang)(yang)(yang)鋒的(de)王重(zhong)(zhong)陽(yang)(yang)(yang)傳(chuan)之以法(fa),將(jiang)(jiang)「先天(tian)(tian)功(gong)」以「一(yi)陽(yang)(yang)(yang)指(zhi)(zhi)」使出,以剋制「蛤(ha)蟆(ma)功(gong)」,以免造成段(duan)(duan)智興(xing)在第一(yi)次「華山論劍」時,身負能(neng)夠(gou)克制「西(xi)毒」歐陽(yang)(yang)(yang)鋒的(de)武(wu)功(gong)卻只是與(yu)「西(xi)毒」戰成平手的(de)古怪結果。王重(zhong)(zhong)陽(yang)(yang)(yang)徒(tu)弟「全真七子」跟師弟「老頑童」周(zhou)伯通均未獲(huo)得(de)傳(chuan)授,僅(jin)與(yu)「南(nan)帝(di)」段(duan)(duan)智興(xing)交換(huan)(huan)過(guo)「一(yi)陽(yang)(yang)(yang)指(zhi)(zhi)」,用意(yi)是要「南(nan)帝(di)」擁有剋制「西(xi)毒」歐陽(yang)(yang)(yang)鋒之能(neng)。「先天(tian)(tian)功(gong)」配合「一(yi)陽(yang)(yang)(yang)指(zhi)(zhi)」除了(le)可以完全對付歐陽(yang)(yang)(yang)鋒外,也可以祛(qu)百(bai)病、調(diao)虛(xu)實,治療沉重(zhong)(zhong)內傷。
在不同小(xiao)說描述(shu)里(li)有(you)(you)不同作(zuo)用(yong),上(shang)(shang)官(guan)鼎小(xiao)說《沉沙谷(gu)》描述(shu):「全(quan)(quan)真(zhen)(zhen)教(jiao)」和(he)「少(shao)林(lin)派(pai)(pai)」是(shi)最敝帚自(zi)珍,天(tian)(tian)下(xia)所矚目的(de)(de)(de)(de)就(jiu)是(shi)「先天(tian)(tian)罡氣(qi)」。全(quan)(quan)真(zhen)(zhen)與少(shao)林(lin)雖(sui)然在取勁上(shang)(shang)有(you)(you)柔剛之(zhi)別,但天(tian)(tian)下(xia)的(de)(de)(de)(de)武(wu)(wu)(wu)學是(shi)萬(wan)變不離其(qi)宗,而以(yi)「人」為(wei)原(yuan)則的(de)(de)(de)(de)。況且,兩(liang)派(pai)(pai)的(de)(de)(de)(de)「先天(tian)(tian)氣(qi)功」又是(shi)同屬一(yi)個范圍的(de)(de)(de)(de)事物。許多「全(quan)(quan)真(zhen)(zhen)派(pai)(pai)」的(de)(de)(de)(de)功訣(jue)上(shang)(shang)不夠份量的(de)(de)(de)(de)地方,拿(na)「少(shao)林(lin)心(xin)法」一(yi)加(jia)對照,便(bian)瞭然於(wu)胸了,而相反的(de)(de)(de)(de),「少(shao)林(lin)心(xin)法」的(de)(de)(de)(de)缺點(dian)也可(ke)以(yi)用(yong)全(quan)(quan)真(zhen)(zhen)之(zhi)長來補救。少(shao)林(lin)和(he)全(quan)(quan)真(zhen)(zhen)佛道兩(liang)門(men)至高武(wu)(wu)(wu)學水乳交融,少(shao)林(lin)和(he)全(quan)(quan)真(zhen)(zhen)的(de)(de)(de)(de)心(xin)法一(yi)旦合并,舉世無敵!使(shi)用(yong)者(zhe):陸介《長干行》描述(shu):流(liu)落(luo)於(wu)關(guan)外「天(tian)(tian)池(chi)派(pai)(pai)」。當年「全(quan)(quan)真(zhen)(zhen)教(jiao)」仗(zhang)以(yi)威攝武(wu)(wu)(wu)林(lin),幾達(da)百馀(yu)年之(zhi)久(jiu),後(hou)來全(quan)(quan)真(zhen)(zhen)沒落(luo),這種(zhong)武(wu)(wu)(wu)功失傳,方有(you)(you)少(shao)林(lin)武(wu)(wu)(wu)當等派(pai)(pai)興起,算起來,這「先天(tian)(tian)功」正(zheng)是(shi)武(wu)(wu)(wu)學之(zhi)源。當掌力達(da)於(wu)極(ji)限(xian)時(shi),能(neng)相觸(chu)無聲(sheng)(sheng),卻最易(yi)傷人內腑,這種(zhong)功夫,有(you)(you)個名稱叫做「否極(ji)泰來」,正(zheng)是(shi)物極(ji)必(bi)反的(de)(de)(de)(de)道理(li),掌力能(neng)不聞一(yi)些(xie)聲(sheng)(sheng)響,空中暗勁橫流(liu),風力激蕩。使(shi)用(yong)者(zhe):「定天(tian)(tian)一(yi)戟(ji)」高戰。