The inward light is one of a group of Bible-related metaphors Quakers use to describe the distinctive way that we experience the Holy Spirit.  Other metaphors include fountain, spring, seed, Christ within and life. This way of experiencing the Holy Spirit is the key to understanding our distinctive practices, like our meetings for worship and business meetings.  When one first becomes aware of the possibility of this kind of experience, it can feel like being born again, to borrow another metaphor from the Bible. Our meeting uses a kind of spiritual companioning called Focusing, to teach and facilitate this unfamiliar experience.

    Paying attention to the Holy Spirit in this way involves felt sensing.  Felt sensing is something that anyone can do but doesn't because they don't know about it or realize its potential.  Felt sensing involves paying attention to inchoate senses of meaning in the body.   Friends locate these sensations in the heart, the bosom, the breast, the reins, the inward parts and mean it literally.  What we sense is how we are affected by situations in our life, in other words, what they mean to us.

It takes time to locate and identify these sensations and to recognize the situations they are connected to.  One does this by turning inward.  So one would typically close their eyes and become silent, which is what you would observe in our meetings for worship. It can also be difficult to stay with the sensations as the mind wanders and one typically has a lot of thoughts about the kinds of situations that come up.  Again it takes time.  As one stays with the sensation though, often the sensation or felt meaning will change and with the change new insight can come or at least come in a way that feels different and fresh.

These changes typically come with a sense of physical release and relief and feel like a gift. Often where one felt stuck in a situation, not knowing how to proceed, one now knows how to move forward.  Friends say that "Way opens."  But again, this takes time and patience and fortitude as one often encounters resistance in trying to sense into all that goes with a particular situation. Think of the Bible story of Jacob wrestling with the angel or Elijah in the cave.

    Because one does not make up the sensations one feels, and because they feel this way and not that way, we can speak of truth with regards to them.  Because we cannot control the changes that occur and because those changes feel like a gift, like grace, we attribute them to the Holy Spirit.

Here are some related quotes from mostly other Quakers.

 For [early Quaker Isaac] Penington, however, sensing is an intuitive, non-rational, feel for something.  And feeling is a way of relating to something that affects us, moves us emotionally.  “Feeling” and “sensing” are, therefore, synonymous.  Both are ways of relating to a reality that involve an emotional awareness.  They are a conscious awareness  that does not use ideas, although we can get an idea about what we are feeling or sensing.  But the idea is not the feeling, though it be filled with the feeling.  Idea is erected upon sense and feeling as their clarification through intellectual content.  Different from modern philosophers, then, Penington affirms that we know realities through conscious awareness that is not conceptual.  Feeling and sensing make it clear that knowing is experiential and not merely having an idea.  This would be nonsense to many modern thinkers but it is startling in its relevance today as some philosophers (existential, phenomenological, feminist, postcritical) turn to experience as their starting point and way of knowing.   R. Melvin Keiser, Knowing the Mystery of Life Within, p. 179

“O dear lambs! Mind the quickenings of life, and the savor and sense which the Lord begets in the heart, and let the outward knowledge (even of what ye have had experience) go, but as the Lord quickens it: and mind not the noises of thoughts and reasonings about things, which the soul’s enemy will be striving to fill you with, and batter you by; but sink down from these, and wait to feel that which lies beneath them; in the free nature, life, virtue, power, and motions whereof alone is your soul’s salvation. . .” Penington, Works, 1863 ed., v. 2, p. 422.

This sounds a lot like Eugene Gendlin in Focusing:
“In this second movement [getting a felt sense] you will probably encounter a lot of static from your mind: self-lectures, analytic theories, cliches, much squawking and jabbering. Somehow you must get down past all that noise to the felt sense underneath.” p. 53.

And a lot like Iro Progoff in At a Journal Workshop: 

We do not at this point “think” of our life, but we “feel” it. We feel its movement in a general and flexible way. We specifically do not think about it, for if we did, we would only have the same thoughts on the subject that we have always had. We know from our experience that the self-analytic, self-judgmental thinking process tends to move in circular grooves, turning in upon itself and repeating itself.
We wish instead to open the way for something new to enter our experience. We therefore do not do what we have been accustomed to doing. We do not think our lives, but we sit in silence and we feel the inner movement of our recent experiences without judgment. We do not direct our thinking, but we let awarenesses present themselves to us regarding this present period of our lives."

Principle is a word that early Friends use over and over again to talk about the inner light. They use it in the way that Aristotle uses it to describe living things:  “It defines the things by whatever in them goes beyond our organizing and doing, whatever in them organizes their activities.” (Eugene Gendlin, “Ultimacy in Aristotle: In Essence Activity,” )    Principle as Friends use the word is closer to the "whatever in them." An example of this is a seed, which controls how a plant will develop, and whose nature or principle we can either nurture and cooperate with or we can stifle by ignoring it.  This principle is independent of us and has a life of its own separate from our own will.  

“How may I do to find the Light in the midst of the darkness of my heart, which is so great, and this Seed so small?
By its discovering and warring against the darkness. There is somewhat which discovereth both the open and secret iniquity of the corrupt Heart, following it under all its coverings of Zeal, Holiness, and all manner of voluntary Humility. . . . and sometimes may cause secret misgivings that all is not well, but there may be a flaw in the covering.”
Penington, Works, 1681 ed., v. 1, p. 57.

“The contemptible means God put into their hands to work this work by: which was not by preaching any new thing, but by directing to a principle which God had already hid in the earth of every man’s heart, and which was to be known by its divine nature and light, turning against and reproving sin; testifying that this was the way the Lord of heaven and earth had chosen, to bring his sons and daughters into the power and glory of his life. Oh! What heart can receive this, what eye can see any beauty in this, but that which the Lord toucheth and openeth! I testify, in the sense of life, that the wisdom of man, yea, the wisdom of Israel corrupted, cannot but despise and turn from this. Is not this the lowest of all dispensations? Is not this common to all mankind?”
Penington, Works, v. 2, p. 375.

“Now the main thing necessary towards the redemption of the soul is, after the revealing of this principle, and some sense and feeling of it, and the turning of the mind towards it, to wait to be made more and more acquainted with it, that in the stirrings, movings, and leadings thereof, there be a ready giving up to be gathered into it, and guided by it.  For though this principle be all life, yet it is at first but as a seed, and the appearance of the Lord in it is but as in a seed; very little, low, weak, hard to be discerned, easy to be overlooked and despised, and some greater and more undeniable appearance expected.”
Penington, Works, v. 2, p. 395.

The Inward Light and Life (or Principle) is corporeal:

“Head notions do but cause disputes; but heart knowledge, heart experience, sense of the living power of God inwardly, the evidence and demonstration of his Spirit in the inward parts, puts an end to disputes, and puts men upon the inward travel and exercise of spirit by that which is new and living, which avails with God.” from “A Brief Account Concerning the People Called Quakers”, by Isaac Penington, p. 357, Volume III, Works.

And as the law came by Moses, so grace and truth came by Jesus Christ. Where is it come? It is come
into thy heart and inward parts." (George Fox, from Early Quaker Writings, edited by Hugh Barbour
and Arthur Roberts, pp. 509-511.)

“Accordingly the living Christian has a certain sense of Divine life in his own breast, which affords him instruction, strength and comfort; in such a manner, as he waits in faithfulness upon it, that he is under no absolute necessity to lean upon the teachings of other men; yet when they come in a degree of the same life, he accepts them as instrumental from God.”
Joseph Phipps, The Original and Present State of Man, 1871 ed.(originally published in 1767), p. 66.

"be inwardly still and attentive to what passes in [your] own breasts. . .” 
Job Scott, "Remarks upon the Knowledge of the One Lord," p. 308.

A Quaker perspective on God from the standpoint of the Inward Life:

"God” is problematic conceived traditionally as a dominating supreme entity, but to image divinity as Life brings God into the centre of our life and hopes. “Life” then is a fruitful way to think about self, world, and God as an interrelated whole.  R. Melvin Keiser, Knowing the Mystery of Life Within, p.128

To centre the spiritual life in these inward depths of life, distinguished from forms (both outward and inward), is to stress the immediacy of the present moment, the vitality of our way of being, and creativity. The reality of spirit and life is not one form among many other forms but is itself formless. It is energy that can issue in forms but is not itself a form. The way we are aware of it is not through ideas which are themselves inward forms that may or may not be filled with the life. We are aware of the life by sensing it, feeling it, abiding in it, being transformed by it. Such awareness is beneath conceiving, although it can be drawn into thought. But expressing such a thought gives no guarantee of life’s continued presence in it.
Life acts on us immediately beneath the ideas we think. Vital in its formless fecundity and freshness, the life is creative, filling us with a sense of deep meaning, emerging into new forms of thought and action, and reinvigorating or reorienting old forms. God acts on us and leads us in creative, not routinised ways, and elicits creativity not conformity in response. That Friends do not live under conventional controls is one of the things that scared people about Quakers early on and that makes them suspicious of us today. We ourselves may want to slip away from the intensity of the immediate presence of the divine to the shelter of modern forms that keep our lives under objective control. The spiritual way of life for Penington is, therefore, focused on a dimension in our personal and social existence beneath the thoughts our minds construct; there we are touched by spiritual energies, drawn forth, challenged, transformed, and brought into the peace and fullness of being. It is this Life that we seek and that we know in the depths of silence and the words emerging from them."   R. Melvin Keiser, Knowing the Mystery of Life Within, pp. 136-137

A Quaker version of being born again in terms of this Inward Life:

"the soul of man hath not only a faculty of cogitation, by which it ordinarily thinks, unites, divides, compares, or forms ideas, but also a latent power of internal sensation, or of perceiving spiritual objects by an inward and spiritual sense, when presented through a proper medium; which, till the beams of Divine light shine upon it, it must be as totally unacquainted with, as the child in its mother’s womb is with its faculties of sight and hearing. . . . Thus born of the spirit, into this proper medium of Divine knowledge, the soul is made acquainted with that spiritual sense it could neither discover, nor believe pertained to it, whilst in its natural state.  This is no new natural faculty added, but its own mental power newly opened and brought into its due place and use.

Words are inadequate to the expression of this internal sense felt in the soul under Divine influence.  It cannot be ideally conveyed to the understanding of the inexperienced; for it is not an image, but a sensation, impossible to be conceived but by its own impression."  Joseph Phipps, The Original and Present State of Man 

“That which God sows and brings up in thee is a sensible plant, not a knowing mind; and thy right judgment is only in the sensibleness of that plant, and not in the understanding or comprehension of thy mind; yea, that sensible plant (which thy wisdom will be very apt to despise and perk over) must batter down and bring to nothing thy understanding, and grow up in the stead of it, if ever thy soul be made a habitation for the life.  Therefore sink into the feeling, and dwell in the feeling, and wait for the savor of the principle of life, and the touches and drawings of the savor, and walk along in it towards the land of life, parting with all, and leaving behind thee, whatever the savor of life disrelisheth; and entering into, and taking up, whatever the savor of life relisheth, that thou mayest be prepared for the Lord, and for the glorious appearance of his Spirit in thee.”

Isaac Penington, Works, v. 2, pp. 396-397.

Grace is an essential component of the Inward Life: 

"The testimonies and declarations which are given forth in obedience to the Lord's requirings, are to bring everyone of you to a sense and feeling of the inward testimony of truth in your own bosoms, to the feeling of the work and operation of the Lord's Spirit upon your hearts, to give to everyone a clear sight and understanding of those things that tend to their souls profit, and to their spiritual advantage and divine growth in grace." Richard Ashby, sermon 1693, 

 “Now what I feel my concern to be, and what I have long felt, and have been endeavouring to spread before this assembly, is, to gather our attention to God within, to the truth within, to the light within, to the grace within, or by whatever name it may be called, for it is the same thing.  For grace may be considered God, light may be considered God; and everything that gives a man a sense of his situation may be considered but as coming from God. So Christ within may be considered but as other terms for God within; for was the Christ within anything but the grace of God? For what was he without, in the personal appearance?  Within, he is nothing but the grace or favour of God unto the children of men.”

Elias Hicks, from a sermon, 12/20/1826.