张景岳曰∶君火者其化虚，相火者其化实。化虚者无形者也，其或衰或旺，惟见于神明。神惟贵足，
衰则可畏也。化实者有形者也，其或热或寒，必着于血气。确有证据，方可以言火也。然君火衰则相火亦败
，此以无形者亏及有形者也；
相火炽则君火亦炎，此亦有形者病及无形者也。
故火得其正，即为阳气，此火不可无，亦不可衰，衰则阳气之虚也；火失其正，是为邪热，此火不可有，
尤不可甚，甚则真阴之败也。然阳以元气言，火以病气言，病在元气者不得以火论。盖人之元气止于充足，
焉得有余？既非有余，何
以言火？所谓无形者其化虚也。惟病在形体者，乃可以察火症，盖其不在气即在血，所谓有形者其火
实也。若以形质之间，本无热症可据，而曰此火也热也，是皆妄谈者矣。且火症即具，犹有虚实之殊，
真假之异，其可不为详辨乎？
虚火病即假热症也。病有寒热真假之不同，真寒宜温，真热宜清，此正治也。而惟假热假寒为难治，
如虚火之病源有二，虚火之外症有四。一曰阴虚者能发热，此以真阴亏损，水不制火也；二曰阳虚者能发热
，此以元阳败竭，火不归源也。此病源之二也。至若外症，一曰阳戴于上，而见于头面咽喉
之间者，此其上虽热而下则寒，所谓无根之火也；二曰阳浮于外，而发于皮肤肌肉之间者，此其外虽热而
内则寒，所谓格阳之火也；三曰阳陷于下，而见于便溺二阴之间者，此其下虽热而中则寒，所谓失位之火也；
四曰阳亢乘阴，而见于精血髓液之间者，此其金水败而铅汞干，所谓阴虚之火也。此外症之四也。
证虽有四，本则惟二，阴虚阳虚尽之矣。第阴虚唯一金水败者是也，治法当壮水，壮水之法只宜甘凉。
阳虚有三，上中下者是也，治宜益火，益火之法只宜温热，大忌清凉。但温热之效速，每一二剂便可奏功；
甘凉之力缓，非多服不能见效也。然清凉之药，终损脾胃，如不得已，易以甘平。倘甘平未效，则惟有甘温之
一法。斯堪实济，否则生气之机，终非清凉所能致也。此义最微，不可不察。
一火有虚实，故热有假真，而察之之法，总当以中气为主，而外寒外热无足据也。故凡假热之证，本
中寒也；假寒之症，本中热也。中寒者原是阴症，中热者原是阳症，内有可据，本皆真病，又何假之有？
张景岳说：君火的化生作用是虚静的，相火的化生作用是实动的。所谓“化虚”，是指其没有具体的形态，它的衰退或旺盛，只体现在人的精神意识上。精神贵在充盈充足，一旦衰退就令人担忧。所谓“化实”，是指它有具体的形态，它的寒或热，必然表现在气血上。只有有确凿的证据，才可以谈论“火”的问题。然而，如果君火衰弱，相火也会随之败坏，这是无形的（神）亏损导致有形的（血气）受损；如果相火炽盛，君火也会炎上，这是有形的（血气）病变波及无形的（神）。
所以，火如果符合正道，就是阳气，这种火是不可缺失的，也不可衰弱，衰弱了就是阳气的虚弱；火如果失去了正道，就成了邪热，这种火是不可存在的，尤其不可过盛，过盛了就会导致真阴的败坏。但是，“阳”是从元气角度说的，“火”是从病气角度说的，病在元气层面的，不能简单地用“火”来论述。因为人的元气只要充足就可以了，哪里会有多余呢？既然不是有余，为什么要说是“火”呢？这就是所说的“无形者其化虚”。只有当病在形体层面时，才能观察火证，因为病不在气就在血，这就是所说的“有形者其火实”。如果在形质之间，本来就没有热症的依据，却硬说这是火、是热，这都是妄谈。况且，即使具备了火证的症状，还有虚实和真假的区别，怎么可以不详细辨别呢？
虚火病就是假热症。疾病有寒热真假的不同，真寒应当用温法，真热应当用清法，这是正治法。而假热假寒最难治疗。例如虚火的病因有两个方面，虚火的外在症状有四种。第一，阴虚的人能发热，这是因为真阴亏损，水不能制约火；第二，阳虚的人也能发热，这是因为元阳败竭，火不能归源。这是病因的两个方面。至于外在症状，第一，阳气戴于上部，表现在头面咽喉之间，这是上面虽然热但下面则寒，称为无根之火；第二，阳气浮越于外部，表现在皮肤肌肉之间，这是外面虽然热但里面则寒，称为格阳之火；第三，阳气陷于下部，表现在大小便及二阴之间，这是下面虽然热但中间则寒，称为失位之火；第四，阳气亢盛乘袭阴分，表现在精血髓液之间，这是金水败坏、铅汞干枯，称为阴虚之火。这是外在症状的四个方面。
症状虽然有四种，但根本原因只有两个，即阴虚和阳虚概括了全部。其中，阴虚特指金水败坏的情况，治疗方法应当壮水，壮水的方法只适宜用甘凉之品。阳虚有三种情况，即上、中、下三焦，治疗方法应当益火，益火的方法只适宜用温热之品，特别忌讳使用清凉药。但是，温热药的见效快，服用一两剂就能奏效；甘凉药的力量缓慢，必须多服才能见效。然而，清凉药终究会损伤脾胃，如果不得已使用，可以改用甘平之药。倘若甘平之药无效，那就只剩下甘温这一种方法了。这才是真正能够救急的，否则，恢复生机的关键，终究不是清凉药所能达到的。这个道理非常微妙，不可以不仔细考察。
火有虚实之分，所以热有真假之别，而辨别的方法，总要以中气（脾胃之气）为主，外部的寒热表现不足以作为依据。所以，凡是假热的证候，本质上是中寒；假寒的证候，本质上是中热。中寒原本属于阴证，中热原本属于阳证，内部有确凿的依据，本质上都是真病，又有什么假的呢？
Zhang Jingyue stated: The transformation of Ministerial Fire (Xiang Huo) is substantial, while that of Sovereign Fire (Jun Huo) is void. That which transforms into the void is formless; its decline or prosperity is manifested only in the spirit and consciousness. The spirit values sufficiency; its decline is to be feared. That which transforms into the substance has form; its heat or cold must manifest in the blood and qi. Only with concrete evidence can one speak of "fire." However, if Sovereign Fire declines, Ministerial Fire also fails; this is a case where the deficiency of the formless affects the substantial. If Ministerial Fire blazes, Sovereign Fire also flares; this is a case where the disease of the substantial affects the formless.
Therefore, when fire is in its proper state, it constitutes Yang Qi (Yang Energy). This type of fire cannot be absent, nor should it be deficient; deficiency here indicates a deficiency of Yang Qi. When fire loses its proper state, it becomes pathogenic heat. This type of fire should not exist, and especially should not be excessive; excess leads to the collapse of True Yin. However, "Yang" refers to primordial qi, whereas "fire" refers to pathogenic qi. Diseases affecting primordial qi cannot simply be discussed as "fire." Human primordial qi needs only to be sufficient; how could there be an excess? Since it is not an excess, why speak of "fire"? This is what is meant by "that which is formless transforms into the void." Only when the disease resides in the physical body can one observe fire syndromes, because the pathology lies either in qi or in blood; this is what is meant by "that which is substantial manifests as substantial fire." If there is no basis for heat symptoms within the physical structure, yet one claims this is fire or heat, such talk is baseless. Moreover, even if fire symptoms are present, distinctions of deficiency/excess and true/false must be made; how can one fail to distinguish them in detail?
Deficient fire disease is synonymous with false heat syndrome. Diseases vary in their true or false nature regarding cold and heat. True cold requires warming, and true heat requires clearing; these are correct treatments. However, false heat and false cold are difficult to treat. For instance, the etiology of deficient fire has two aspects, and its external manifestations have four types. First, yin-deficient individuals may experience fever due to the depletion of True Yin, resulting in water's inability to control fire. Second, yang-deficient individuals may also experience fever due to the exhaustion of Primordial Yang, causing fire to fail to return to its source. These constitute the two etiologies. Regarding external manifestations: first, yang floats upward, appearing between the head, face, and throat; although the upper part is hot, the lower part is cold, known as "rootless fire." Second, yang floats outward, appearing between the skin and muscles; although the exterior is hot, the interior is cold, known as "repelling yang fire." Third, yang sinks downward, appearing in the bowels, bladder, and genitalia; although the lower part is hot, the middle part is cold, known as "displaced fire." Fourth, yang hyperactivity attacks yin, appearing in essence, blood, marrow, and fluids; this indicates the failure of Metal and Water, and the drying up of Lead and Mercury, known as "yin-deficient fire." These constitute the four external manifestations.
Although there are four symptom patterns, the root causes are only two, encompassing both yin and yang deficiency. Specifically, yin deficiency refers to the failure of Metal and Water; the treatment method should be to strengthen water, employing only sweet-cool agents. Yang deficiency has three subtypes corresponding to the upper, middle, and lower jiao; the treatment method should be to benefit fire, employing only warm-hot agents, strictly avoiding cooling agents. However, warm-hot agents act quickly, often achieving results after one or two doses; sweet-cool agents act slowly, requiring prolonged administration to show effects. Nevertheless, cooling agents ultimately damage the Spleen and Stomach. If unavoidable, they should be replaced with sweet-neutral agents. If sweet-neutral agents prove ineffective, the only remaining option is the sweet-warm method. This approach truly saves lives; otherwise, the mechanism of generating vitality cannot be achieved by cooling agents. This principle is subtle and must be carefully examined.
Since fire has deficiency and excess, heat has false and true aspects. The method of differentiation always prioritizes Middle Qi (Spleen and Stomach Qi), as external cold or heat is insufficient as evidence. Therefore, all false heat syndromes essentially involve internal cold; all false cold syndromes essentially involve internal heat. Internal cold originally belongs to Yin syndromes, and internal heat originally belongs to Yang syndromes. With concrete internal evidence, these are fundamentally true diseases, leaving no room for falsehood.